System and method for a radio frequency filter

ABSTRACT

In accordance with an embodiment, a method of operating an RF system includes filtering a first wideband RF signal using a wideband filter bank. Filtering the first RF signal includes separating the first wideband RF signal into frequency cluster signals, where each frequency cluster signal of the frequency cluster signals includes different frequency ranges, the first wideband RF signal includes multiple RF bands, and each of the different frequency ranges comprises a plurality of RF bands of the multiple RF bands. The method further includes band stop filtering at least one of the frequency cluster signals to produce a band stopped frequency cluster signal.

This application claims the benefit of U.S. Provisional Application No.62/595,898, filed on Dec. 7, 2017, and the benefit of U.S. ProvisionalApplication No. 62/641,664, filed on Mar. 12, 2018, and claims priorityto European Application No. 18210361.4, filed Dec. 5, 2018 that alsoclaims priority to U.S. Provisional Applications 62/595,898 and62/641,664, which applications are hereby incorporated herein byreference in their entireties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to the following co-pending and commonlyassigned patent applications: Attorney Docket Number INF 2018 P 51521US,U.S. patent application Ser. No. ______, filed on Dec. 5, 2018; AttorneyDocket Number INF 2018 P 51522US, U.S. patent application Ser. No.______, filed on Dec. 5, 2018; Attorney Docket Number INF 2018 P51523US, U.S. patent application Ser. No. ______, filed on Dec. 5, 2018,Attorney Docket Number INF 2018 P 51524US, U.S. patent application Ser.No. ______, filed on Dec. 5, 2018, Attorney Docket Number INF 2018 P51527US, U.S. patent application Ser. No. ______, filed on Dec. 5, 2018,Attorney Docket Number INF 2018 P 51528US, U.S. patent application Ser.No. ______, filed on Dec. 5, 2018, Attorney Docket Number INF 2018 P51529US, U.S. patent application Ser. No. ______, filed on Dec. 5, 2018,Attorney Docket Number INF 2018 P 51842US, U.S. patent application Ser.No. ______, filed on Dec. 5, 2018, Attorney Docket Number INF 2018 P51843US, U.S. patent application Ser. No. ______, filed on Dec. 5, 2018,which applications are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present invention relates generally to a system and method, and, inparticular embodiments, to a system and method for a radio frequencyfilter.

BACKGROUND

In many RF Systems, such as portable wireless devices, more than onereceive frequency or transmit frequency may be simultaneously active ina single radio device. When the respective frequency bands are far awayfrom each other and/or when the frequency bands are processed withdifferent gains, different frequency channels are separated in thefrequency domain and processed in separate signal paths. Such frequencybands may include frequency bands devoted to different bands of a sametelecommunication transmission standard, different bands devoted todifferent telecommunications standards (such as LTE and GSM), as well asmultiple bands devoted to different service types (such as cellularcommunication, WiFi and GPS). Many systems today require flexiblefrequency planning and simultaneous processing of more than twochannels, which makes a fixed frequency de-multiplexer filter design(with n frequency bands) challenging to design.

The growing complexity in RF front ends (e.g. due to the growing numberof supported bands) results in higher insertion loss, reduced referencesensitivity and significantly increased area with respect to physicallyimplementing the front end. For example, the implementation of an RFfront end that operates over multiple frequency bands may includemultiple fixed filters that are switched in and out of the RF signalpath depending on the particular operation mode of the radio or on aparticular carrier aggregation use case. In such systems, a greaternumber of switched filters are used to support a greater number ofcarrier aggregation use cases.

SUMMARY

In accordance with an embodiment, a method of operating an RF systemincludes filtering a first wideband RF signal using a wideband filterbank. Filtering the first RF signal includes separating the firstwideband RF signal into frequency cluster signals, where each frequencycluster signal of the frequency cluster signals includes differentfrequency ranges, the first wideband RF signal includes multiple RFbands, and each of the different frequency ranges comprises a pluralityof RF bands of the multiple RF bands. The method further includes bandstop filtering at least one of the frequency cluster signals to producea band stopped frequency cluster signal.

In accordance with a further embodiment, an RF system includes awideband filter bank having an input and a plurality of outputs. Thewideband filter bank is configured to separate a wideband RF signal atan input of the wideband filter bank into a plurality of frequencyclusters at the plurality of outputs of the wideband filter bank, whereeach frequency cluster of the plurality of frequency clusters includes adifferent frequency range, and each frequency range covers a pluralityof RF bands of the wideband RF signal. The RF system further includes atleast one band stop filter having an input coupled to one of theplurality of outputs of the wideband filter bank.

In accordance with another embodiment, an RF system includes a first RFfilter having a first input configured to be coupled to an antenna,where the first RF filter configured to provide a first bandpassresponse passing a first frequency band from the first input to a firstbandpass output, and a first band stop response rejecting the firstfrequency band from the first input to a first band stop output; ann-plexer having an input coupled to the first band stop output of thefirst RF filter; and a first tunable band stop filter coupled to anoutput of the n-plexer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a schematic of an exemplary RF front end circuit;

FIGS. 2A to 2D illustrate block diagrams of embodiment RF front endcircuits;

FIG. 3A illustrates an embodiment frequency cluster plan; FIG. 3Billustrates a table of frequency channels supported by the embodimentfrequency cluster plan of FIG. 3A; FIG. 3C illustrates an embodiment RFfront-end circuit that implements a portion of the embodiment frequencycluster plan shown in FIG. 3A; and FIGS. 3D-3N illustrate forwardtransmission diagrams that show the performance of various portions ofthe RF front-end circuit of FIG. 3B;

FIG. 4A illustrates an alternative frequency cluster plan; and FIG. 4Billustrates a further embodiment RF front-end circuit that implementsthe alternative frequency cluster plan of FIG. 4A; FIG. 4C illustrates afurther alternative frequency cluster plan; FIG. 4D illustrates aconventional implementation of the frequency cluster plan of FIG. 4C;and FIG. 4E illustrates an embodiment RF front-end circuit thatimplements the alternative frequency cluster plan of FIG. 4C;

FIGS. 5A and 5B illustrate block diagrams of exemplary RF systems;

FIGS. 6A to 6G illustrate block diagrams of embodiment RF systems; andFIGS. 6H-6L illustrate schematics of bandpass LNAs that can be used toimplement the embodiment RF systems of FIGS. 6C, 6F and 6G;

FIGS. 7A-7N, 8A-8E, 9A-9C, 10A-10G, 11A-11C and 12A-E and 13A-13Eillustrate various filter structures that can be used to implement thevarious filters utilized in embodiments of the present invention;

FIG. 14 illustrates a block diagram of an embodiment RF front-endsystem;

FIGS. 15A and 15B illustrate conventional embodiments of multi-bandmulti-transceiver front end circuits;

FIGS. 16A and 16B illustrate multi-band multi-transceiver front endcircuits according to embodiments of the present invention;

FIGS. 17A-17C illustrate block diagram of embodiments RF systems thatinclude a combined receive/transmit antenna and a duplexer, tunablenotch filters in the transmit path and tunable filters in the receivepath;

FIGS. 18A-18D illustrate embodiment RF systems having a combinedreceive/transmit antenna, tunable notch filters in the transmit path andin the receive path and adjustable phase shifters/matching networks tocombine the transmit path and the receive path;

FIGS. 19A-19C illustrate embodiment RF systems in which isolationbetween the transmit path and the receive path is achieved by usingseparate transmit and receive antennas;

FIG. 20A illustrates embodiment RF systems directed to multi-transmittersystems that have more than one transmitter active at the same timecoupled to the same physical antenna; and FIG. 20B illustratesembodiment RF systems directed to multi-transmitter systems that havemore than one transmitter active at the same time and one (can be morethan one) receive Path coupled to the same physical antenna;

FIGS. 21A-21B illustrate embodiment RF systems directed to time divisionduplex (TDD) systems;

FIG. 22 illustrates a table depicting embodiments transmit/receivepath/combining structures and corresponding transmit/receive path filterconfigurations;

FIG. 23 illustrates an embodiment TDD RF system;

FIG. 24A illustrates an embodiment RF system utilizing tunable bandpassfilters and adjustable phase shifters/matching networks; and FIGS. 24B-Dillustrate graphs showing the selectivity and return loss of the RFsystem of FIG. 24A;

FIG. 25 illustrates an embodiment RF system utilizing tunable bandpassfilters and a circulator;

FIG. 26A illustrates an embodiment RF system utilizing tunable bandpassfilters and a quadrature combiner; and FIG. 26B illustrates graphsshowing the selectivity and insertion loss of the RF system of FIG. 26A;and

FIG. 27 illustrates an embodiment RF system utilizing tunable bandpassfilters, adjustable phase shifters/matching networks; and separatetransmit and receive antennas.

Corresponding numerals and symbols in different figures generally referto corresponding parts unless otherwise indicated. The figures are drawnto clearly illustrate the relevant aspects of the preferred embodimentsand are not necessarily drawn to scale. To more clearly illustratecertain embodiments, a letter indicating variations of the samestructure, material, or process step may follow a figure number.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferredembodiments in a specific context, a system and method for RF filteringat a front-end of a wireless communications system. Embodiments of theinvention may also be applied to other RF-based systems including, butnot limited to radar systems, high frequency wireline communicationssystems.

Current solutions apply arrays of dedicated Carrier Aggregation (CA) RFFilter Arrays that include two or more multiplexed RF filters, which areswitched in to support the specific Carrier Aggregation use cases. Thus,bands that support different CA cases may experience redundancy inseveral CA RF Filter arrays. This design methodology increases thenumber of RF switches in the system, increases filter content and areain a product and complicates the design effort for new products withdifferent CA combinations. Such a design methodology may result inredundant filter circuits when the same frequency band is supported byCA combinations. In addition, system losses may increase as more CAcases are supported. These increased system losses may lead todegradation in reference sensitivity in some systems.

FIG. 1 illustrates an example of a conventional RF front end 100 thatutilizes RF filters and dedicated RF band filters. As shown, RF frontend 100 includes antenna 110 that is coupled to multiple RF filters 108via multiple switching circuits 102. The output of filters 108 arecoupled to LNAs 106 via additional switching circuits 102. RF filters108 may include for example fixed bandpass filters, diplexers,triplexers or other types of RF filters. The selection of the number andtype of filters 108 that are used in a particular system are based onthe particular characteristics of the RF system as well as the RFenvironment in which the particular RF system operates. In somesituations, the particular RF system may be subject to some frequenciesin which high amplitude RF signals are present, for example, in the caseof a transmitter that is present within the RF system that may betransmitting at the same time that the RF front end is receiving. Insuch cases, the frequency plan of RF front-end 100 and the selection offilters 108 would take into account the presence of the high amplitudetransmitting signal and the filter characteristics of filters 108 wouldbe designed to sufficiently reject the high amplitude transmittingsignal. As can be seen, as more combinations of different RFenvironments are supported by RF front-end 100, more RF switches 102 andRF filters 108 are used to support these combinations. As a result, thesize and complexity of RF system 100 may increase according to thenumber of Carrier Aggregation use cases that are supported.

In embodiments of the present invention, an RF front end utilizes wideband RF filters that group various Carrier Aggregation combinations intofrequency clusters. In some embodiments, the cluster structure isselected such that a single band operates in each particular cluster.However, a single band in one cluster may operate simultaneously withother single bands in other clusters. These clusters are separated fromeach other via wideband cluster filters, such that interference from onecluster does not jeopardize the RF performance of any band in anothercluster.

In some embodiments, transmit activity that occurs within the samecluster as a receive channel may act as a strong interferer, such as inthe case of frequency division duplex (FDD). In such embodiments, RFband stop filters may be used to suppress the transmit signal. However,in embodiments in which transmission does not occur at the same time asreception, such as in time division duplex (TDD) systems, such band stopfilters may be bypassed or omitted to achieve lower insertion loss.

FIG. 2A illustrates an RF system 200 according to an embodiment of thepresent invention. As shown, RF system 200 includes an antenna 201coupled to wideband filter bank 202. In various embodiments, widebandfilter bank 202 contains a plurality of wideband filter functions. Inthe illustrated embodiment four bands are shown. The first band has afrequency range of f1A to f1B, the second band has a frequency range off2A to f2B, the third band has a frequency range of f3A to f3B, and thefourth band has a frequency range of f4A to f4B. Each of these frequencybands represents a frequency cluster that includes multiple transmissionand/or reception frequencies. It should be understood that whilewideband filter bank 202 is shown having four frequency clusters, inalternative embodiments of the present invention any number of frequencyclusters may be supported. In the illustrated example, a stop band isshown between the second and third frequency bands. However, inalternative embodiments, all of the frequency bands may be directlyadjacent to each other, all of the frequency bands may have stop bandsbetween them, or the frequency bands may be distributed with anycombination being directly adjacent to each other or having stop bandsbetween them. For example, each frequency band may have its owndedicated lower corner frequency f_(low) and its own dedicated uppercorner frequency f_(high).

As shown, wideband filter bank 202 is coupled to RF transceiver 208. RFtransceiver 208 may include circuitry that supports the transmissionand/or reception of radio frequency signals. Such circuitry may include,for example low noise amplifiers (LNAs) that are used to amplify RFsignals received by antenna 201. As shown, tunable band reject filter204 is coupled between the fourth cluster of wideband filter bank 202and fourth input of RF transceiver 208. Tunable band reject filter 204may be used, for example, to filter out strong interferer such assignals that are transmitted by RF system 200. Similarly, tunable bandreject filter 206 is coupled between the second cluster of widebandfilter bank 202 and second input of RF transceiver 208. While only twoband reject filters 204 and 206 supporting two frequency clusters aredepicted in FIG. 2A, it should be understood that any number offrequency channels may include one or more tunable band reject filtersdepending on the particular system and its specifications. In someembodiments fixed frequency band reject filters may be used in place oftunable band reject filters. Fixed frequency band reject filters may beused, for example, in systems in which the transmission frequency isfixed.

FIGS. 2B-2D illustrate example filter implementations that may be usedto implement wideband filter bank 202 shown in FIG. 2A. FIG. 2Billustrates filter bank 210 that is implemented using a cascade ofextractors 212, 214 and 216. As shown, extractors 212, 214 and 216 havea bandpass response between ports 1 and 2, and a band stop responsebetween ports 1 and 3. During operation, extractor 212 produces thefiltered output of the first band (Band₁) using its bandpass function,and passes the remaining frequency bands (except for the first band)using its band reject function. Similarly, extractor 214 produces thefiltered output of the second band (Band₂) using its bandpass function,and passes the remaining frequency bands (except for the second band)using its band reject function. Extractor 216 produces the filteredoutput of the next to last band (Band(n−1)) using its bandpass function,and passes the last remaining frequency band Band(n) using its bandreject function. Any number of extractors can be cascaded together.Extractor 216 may be implemented using filter circuits known in the artincluding structures as SAW filters, BAW filters, FBAR filters, looselycoupled resonators, LC filters, LC resonators, tunable LC filters,microstrip filters or other filter structures. In some embodiments,extractors 212, 214 and 216 may be implemented using isolated filtercore structures disclosed in U.S. patent application Ser. No.14/874,256, which application is incorporated herein by reference in itsentirety. It should be understood that while extractors 212, 214 and 216are shown using bandpass filter functions, other filter functions couldbe used such as lowpass, highpass and bandstop filter functions.

In some embodiments, wide band filter bank 202 may be implemented usingparallel bandpass filters as shown in FIG. 2C, which illustrates filterbank 220 that includes parallel bandpass filters 222, 224 and 226. Wideband filter bank 202 may also be implemented using multi-port filtersuch multi-port filter 230 shown in FIG. 2D. As shown multi-port filter230 is an n-plexer that is configured to pass a different band betweenport 1 and each of the remaining ports 1 through n+1 Such an n-plexermay be used to enable an embodiment system to receive n frequency bandsor n frequency clusters. For example, multi-port filter 230 passes thefirst band between ports 1 and 2, the second band between ports 1 and 3,and the nth band between ports 1 and n+1. In various embodiments,multi-port filter 230 may be implemented using various n-plexercircuitry known in the art including SAW filters, BAW filters, FBARfilters, loosely coupled resonators, LC filters, LC resonators, tunableLC filters, microstrip filters or other filter structures. In someembodiments, multi-port filter 230 may be implemented using isolatedfilter core structures described, for example, with respect to FIGS.2A-2D and 3A-3D in U.S. patent application Ser. No. 14/874,256, whichapplication is incorporated herein.

It should be further understood that wide band filter bank 202 may alsobe implemented using a combination of cascaded diplexer, parallelbandpass filters and/or multi-port filters to achieve the desired filterclusters. In some embodiments, wide band filter bank 202 may also beadapted to include narrowband filter functions in addition to thewideband cluster filters.

FIGS. 3A to 3N illustrate a specific example implementation of a RFfront-end filter system that could be used in a portable cellulardevice. In some embodiments, the RF front-end filter system described inFIGS. 3A to 3N may be used to implement RF system 200 shown in FIG. 2A.It should be understood that the embodiment of FIGS. 3A to 3N is justone example of many possible ways to implement an RF front-end filtersystem according to embodiments of the present invention.

FIG. 3A illustrates a block diagram of an embodiment frequency clusterplan. As shown the frequency cluster plan includes cluster A that has afrequency range from 703 MHz to 821 MHz; cluster B that has a frequencyrange from 853 MHz to 960 MHz; cluster C that has a frequency range from1475.9 MHz and 1559.0 MHz; cluster D that has a frequency range from1805 MHz to 2025 MHz; cluster E that has a frequency range from 2100 MHzto 2200 MHz; cluster Fl that has a frequency range from 2300 MHz to 2400MHz; a Wi-Fi cluster that has a frequency range from 2400 MHz to 2483MHz; cluster Fh that has a frequency range from 2496 MHz to 2690 MHz;and cluster G that has a frequency range from 3400 MHz to 3800 MHz.Because frequency cluster D is located very close to frequency clusterC, the low end of the filter used to implement frequency cluster D has asteep frequency rolloff as indicated by rectangle 302. Similarly,because frequency cluster E is located very close to frequency clusterD, the low end of the filter used to implement frequency cluster E has asteep rolloff as indicated by rectangle 304. In some embodiments, thecluster filter used to implement the Wi-Fi band also has a steep rolloff because of the narrow frequency of the Wi-Fi band. This steep rolloff is due to the small frequency guard spectrum between the Wi-Fi bandand the cellular bands.

In the illustrated embodiment, clusters D, Fl, and Fh representfrequency bands in which there is transmit activity. Accordingly, bandstop filters are used to reject the interferers within these bands. Insome embodiments, these band stop filters may be cascaded after thefilter used to filter the particular cluster. These band stop filtersare represented by blocks 306 and 308 within cluster D, block 309 withincluster Fl, and block 311 within cluster Fh. Alternatively, cluster Dmay include a single band stop filter. In some embodiments, a variablefrequency band stop filter, such as a tunable notch filter, may be usedin order to increase system flexibility. Alternatively, fixed frequencynotch filters may be used when appropriate.

FIG. 3B illustrates a table of example frequency bands that may besupported by the frequency plan shown in FIG. 3A. As shown in FIG. 3Bthe entries in the table of example frequency bands include a bandnumber, a transmission type, an uplink frequency (UL) range thatrepresents the frequencies being transmitted by the system, and adownlink frequency (DL) range that represents the frequencies beingreceived by the system. Example transmission types include, for example,frequency division duplex (FDD) and time division duplex (TDD). Itshould be understood that in alternative embodiments, differentfrequency bands as well as different transmission/reception types may besupported. For example, in some embodiments, some clusters may supportonly signal reception or signal transmission.

FIG. 3C illustrates RF front-end 310 that can be used to implementclusters D, E, Fl, Wi-Fi and Fh of FIG. 3A. RF front-end 310 includes acascade of RF filters 312, 316 and 319 that are used to filter out thevarious clusters. More specifically, RF filter 312 is a three-portfilter that has a bandpass response between port 1 and port 3, and aband stop response between port 1 and port 2. Accordingly, RF filter 312passes the Wi-Fi band between antenna 340 and the cluster Wi-Fi outputof RF front end 310, and rejects the Wi-Fi band between antenna 340 andthe remaining cluster filters 316 and 319. In other words, RF filter 312removes or attenuates the Wi-Fi band from the signal received by antenna340 prior to filtering by RF filter 316. RF filter 312 may beimplemented using a three-port extractor cluster filter. This filter maybe implemented using various diplexer circuits as discussed above. Insome embodiments, RF filter 312 may be bypassed with 314 when the Wi-Fisystem is deactivated or not in use. Bypassing RF filter 312 may improvethe noise performance of RF front-end 310 by reducing the attenuation ofthe filter.

RF filter 316 is used to separate out the frequencies of cluster E fromclusters D, Fl and Fh. As shown, RF filter 316 is also a three portfilter that has a bandpass response between port 1 and port 3, and aband stop response between port 1 and port 2. Accordingly, RF filter 316passes the Cluster E band between antenna 340 and the Cluster output ofRF front end 310, and rejects the Cluster E band between antenna 340 andthe remaining cluster filter 319. RF filter 316 may be bypassed when RFfront-end 310 is not receiving or transmitting RF signals within thefrequency band of cluster E. RF filter 316 may be implemented using athree-port extractor cluster filter and may be implemented using variousdiplexer circuits as discussed above.

In some embodiments, RF filters 312 and 316 may be implemented as highlyselective filters. Triplexer-based implementations of RF filters 312 and316 may provide high selectivity in such cases. Alternatively, a hybridextractor, such as that described with respect to FIG. 4B of U.S. patentapplication Ser. No. 14/874,256, may be used to implement RF filter 312and/or 316. In such cases, the high side and/or the low side filterslope may be enhanced by using tuned or switched filter slope.

In embodiments in which WiFi filtering is not utilized, the low sidetunable/switchable filter slope of RF filter 312 can be implemented as asteeper high side filter slope in the filtering of cluster Fl, and thehigh side tunable/switchable filter slope of RF filter 312 can beimplemented as a steeper low side filter slope in the filtering ofcluster Fh. For example, RF filter 312 may be implemented using atunable slope enhancer with bypass for improved selectivity on low sideof the WiFi band at 2400 MHz and/or on the high side of the WiFi band at2483 MHz; and RF filter 316 may be implemented using a tunable slopeenhancer on the low side of the band to provide improved selectivity onbands 23 and 65.

RF filter 319 separates the remaining frequencies of clusters D, Fl andFh. As shown, RF filter 319 is implemented as a four port filter havingvarious bandpass responses corresponding to the frequency of clusters D,Fl and Fh. In some embodiments, RF filter 319 is implemented usingtriplexer filter structures known in the art. Alternatively, RF filter319 may be implemented using an extractor cluster filter to filter outcluster Fl followed by a diplexer to filter cluster D and cluster Fh. Insome embodiments, RF filter 319 may be implemented using a tunable slopeenhancer with bypass for improved selectivity on the UL channel of bands3 and 9.

In various embodiments, the frequencies of frequency bands D, Fl and Fhare further filtered using band stop filters 330, 326, and 322 in orderto attenuate strong interferers or signals being transmitted by thesystem itself within its respective frequency cluster. Band stop filters330, 326, and 322 may be implemented using band reject filter structuresand/or tunable band stop filter structures known in the art. In someembodiments, band stop filters 330, 326, and 322 may be implementedusing frequency tunable BAW resonators disclosed in U.S. ProvisionalApplication Nos. 62/595,898 and 62/641,664, as well as co-pending U.S.patent application Ser. No. ______ corresponding to docket numbersINF-2018-P-51523US entitled, “Acoustically Coupled Resonator Notch andBandpass Filters,” INF-2018-P-51524US entitled, “Acoustically CoupledResonator Notch and Bandpass Filter,” INF-2018-P-51527US entitled,“Tunable Resonator Element, Filter Circuit and Method,”INF-2018-P-51528US entitled, “Tunable Resonator Element, Filter Circuitand Method,” and INF-2018-P-51529US entitled, “Acoustically CoupledResonator Notch and Bandpass Filters,” which applications areincorporated herein by reference in their entirety.

In some embodiments, band reject filters 330, 326 and 322 may bebypassed with switches 332, 328 and 324, respectively. Switches 314,318, 324, 328 and 332 may be implemented using RF switch structuresknown in the art. Tunable band stop filter 322 with bypass switch 324may provide improved selectivity for the UL channel of band 7; tunableband stop filter 326 with bypass switch 328 may provide improvedselectivity for the on band 30; and tunable band stop filter 330 withbypass switch 332 may provide improved selectivity for the UL channelsof bands 2, 25, 1 and 25 (for CA cluster 1).

FIG. 3D illustrates a graph of the passband response of RF filter 312between ports 1 and 3. Curve 350 illustrates the passband response in dBaccording to the ordinate axis on the left of the graph, and curve 352illustrates a magnified view of the passband response in dB according tothe ordinate axis on the right of the graph.

FIG. 3E illustrates a graph of the passband response of RF filter 312between ports 1 and 2. Curve 354 illustrates the passband response in dBaccording to the ordinate axis on the left of the graph, and curve 356illustrates a magnified view of the passband response in dB according tothe ordinate axis on the right of the graph.

In embodiments that do not support WiFi, the system of FIG. 3C can bemodified by replacing RF filter 312 with a two-port tunable band stopfilter that is coupled between antenna 340 and port 1 of RF filter 316.This two-port tunable band stop filter is used, for example, toattenuate RF signals transmitted by nearby WiFi devices. FIG. 3Fillustrates a graph of the passband response of such a two-port tunableband stop filter. Curve 358 illustrates the passband response in dB at afirst setting (center frequency of 2.47 GHz) according to the ordinateaxis on the left of the graph, and curve 362 illustrates a magnifiedview of the passband response in dB at the first setting (centerfrequency of 2.47 GHz) according to the ordinate axis on the right ofthe graph. Curve 360 illustrates the passband response in dB at a secondsetting (center frequency of 2.42 GHz) according to the ordinate axis onthe left of the graph, and curve 364 illustrates a magnified view of thepassband response in dB at the second setting (center frequency of 2.42GHz) according to the ordinate axis on the right of the graph.

FIG. 3G illustrates a graph of the passband response of RF filter 316between ports 1 and 3. Curve 366 illustrates the passband response in dBaccording to the ordinate axis on the left of the graph, and curve 368illustrates a magnified view of the passband response in dB according tothe ordinate axis on the right of the graph.

FIG. 3H illustrates a graph of the passband response of RF filter 316between ports 1 and 2. Curve 370 illustrates the passband response in dBaccording to the ordinate axis on the left of the graph, and curve 372illustrates a magnified view of the passband response in dB according tothe ordinate axis on the right of the graph.

FIG. 3I illustrates a graph of the passband response of RF filter 319between ports 1 and 2, which are used to extract cluster Fh. Curve 374illustrates the passband response in dB according to the ordinate axison the left of the graph, and curve 376 illustrates a magnified view ofthe passband response in dB according to the ordinate axis on the rightof the graph.

FIG. 3J illustrates a graph of the passband response of RF filter 319between ports 1 and 3, which are used to extract cluster Fl. Curve 378illustrates the passband response in dB according to the ordinate axison the left of the graph, and curve 380 illustrates a magnified view ofthe passband response in dB according to the ordinate axis on the rightof the graph.

FIG. 3K illustrates a graph of the passband response of RF filter 319between ports 1 and 4, which are used to extract cluster D. Curve 382illustrates the passband response in dB according to the ordinate axison the left of the graph, and curve 384 illustrates a magnified view ofthe passband response in dB according to the ordinate axis on the rightof the graph.

FIG. 3L illustrates graphs of passband responses of tunable band rejectfilter 322 that is used to reject interferers from cluster Fh. Curve 386illustrates the passband response in dB at a first setting (centerfrequency of 2.51 GHz) according to the ordinate axis on the left of thegraph, and curve 388 illustrates a magnified view of the passbandresponse in dB at the first setting (center frequency of 2.51 GHz)according to the ordinate axis on the right of the graph. Curve 390illustrates the passband response in dB at a second setting (centerfrequency of 2.57 GHz) according to the ordinate axis on the left of thegraph, and curve 392 illustrates a magnified view of the passbandresponse in dB at the second setting (center frequency of 2.57 GHz)according to the ordinate axis on the right of the graph.

FIG. 3M illustrates graphs of passband responses of tunable band rejectfilter 326 that is used to reject interferers from cluster Fl. Curve 392illustrates the passband response in dB at a first setting (centerfrequency of 2.18 GHz) according to the ordinate axis on the left of thegraph, and curve 394 illustrates a magnified view of the passbandresponse in dB at the first setting (center frequency of 2.51 GHz)according to the ordinate axis on the right of the graph. Curve 396illustrates the passband response in dB at a second setting (centerfrequency of 2.29 GHz) according to the ordinate axis on the left of thegraph, and curve 398 illustrates a magnified view of the passbandresponse in dB at the second setting (center frequency of 2.29 GHz)according to the ordinate axis on the right of the graph.

FIG. 3N illustrates graphs of passband responses of tunable band rejectfilter 330 that is used to reject interferers from cluster D. Curve 391illustrates the passband response in dB at a first setting (centerfrequency of 1.84 GHz) according to the ordinate axis on the left of thegraph, and curve 393 illustrates a magnified view of the passbandresponse in dB at the first setting (center frequency of 184 GHz)according to the ordinate axis on the right of the graph. Curve 397illustrates the passband response in dB at a second setting (centerfrequency of 1.91 GHz) according to the ordinate axis on the left of thegraph, and curve 397 illustrates a magnified view of the passbandresponse in dB at the second setting (center frequency of 1.91 GHz)according to the ordinate axis on the right of the graph.

FIGS. 4A-4E illustrate further frequency plans and RF-end circuitsaccording to alternative embodiments of the present invention. Theembodiment of FIGS. 4A and 4B is similar to the embodiment of FIGS.3A-3N with the exception that RF filter 312 used to extract the WiFiband is eliminated and WiFi selectivity is added to the filtering ofclusters Fh and Fl. In some embodiments, the RF front-end filter systemdescribed in FIGS. 4A to 4E may be used to implement RF system 200 shownin FIG. 2A.

As shown in FIG. 4A, the frequency cluster plan includes cluster D thathas a frequency range from 1805 MHz to 2025 MHz; cluster E that has afrequency range from 2100 MHz to 2200 MHz; cluster Fl that has afrequency range from 2300 MHz to 2400 MHz; and cluster Fh that has afrequency range from 2496 MHz to 2690 MHz. Because frequency cluster Dis located very close to other channels in frequency cluster C (notshown), the low end of the filter used to implement frequency cluster Dhas a steep frequency rolloff as indicated by rectangle 302. Similarly,because frequency cluster E is located very close to frequency clusterE, the low end of the filter used to implement frequency cluster E has asteep rolloff as indicated by rectangle 304; and because frequencycluster Fl is located very close to frequency cluster E, the low end ofthe filter used to implement frequency cluster Fl has a steep rolloff asindicated by rectangle 420.

Because a WiFi extraction filter is not used in the embodiment of FIGS.4A and 4B, the high end of the filter used to implement frequencycluster Fl has a steep rolloff as indicated by rectangle 422; and thelow end of the filter used to implement frequency cluster Fh has a steeprolloff as indicated by rectangle 424 in order to provide rejection ofWiFi signals in the WiFi band.

In the illustrated embodiment, clusters D, Fl, and Fh representfrequency bands in which there is transmit activity. Accordingly, bandstop filters are used to reject the interferers within these bands. Insome embodiments, these band stop filters are cascaded after the filterused to filter the particular cluster. These band stop filters arerepresented by block 306 within cluster D and block 311 within clusterFh.

FIG. 4B illustrates RF front-end 400 that can be used to implement thefrequency plan of FIG. 4A. RF front-end 400 includes a cascade of RFfilters 316 and 402 that are used to filter out the various clusters. Asdescribed above with respect to FIG. 3C, RF filter 316 is used toseparate out the frequencies of cluster E from clusters D, Fl and Fh. Asshown, RF filter 316 is a three port filter that has a bandpass responsebetween port 1 and port 3, and a band stop response between port 1 andport 2. Accordingly, RF filter 316 passes the Cluster E band betweenantenna 340 and the Cluster E output of RF front end 400, and rejectsthe Cluster E band between antenna 340 and the remaining cluster filter402. RF filter 316 may be bypassed with switch 318 when RF front-end 400is not receiving or transmitting RF signals within the frequency band ofcluster E. In various embodiments, RF filter 316 is implemented using atunable slope enhancer on the low end with bypass for improvedselectivity of bands 23 and 65.

RF filter 402 separates the remaining frequencies of clusters D, Fl andFh. As shown, RF filter 402 is implemented as a four port filter havingvarious bandpass responses corresponding to the frequencies of clustersD, Fl and Fh. In some embodiments, RF filter 402 is implemented usingtriplexer filter structures known in the art. Alternatively, RF filter402 may be implemented using an extractor cluster filter to filter outcluster Fl followed by a diplexer to filter cluster D and cluster Fh.

In some embodiments, the rolloff of the various bands of RF filter 402is enhanced using tunable slope enhancers. For example, the low side ofthe high frequency band of RF filter 402 used to extract cluster Fh maybe implemented using a tunable slope enhancer for improved selectivityat the high end of the WiFi band at 2483 MHz; the low side of the middlefrequency band of RF filter 402 used to extract cluster Fl may beimplemented using a tunable slope enhancer for improved selectivity onband 30; the high side of the middle frequency band of RF filter 402used to extract cluster Fl may be implemented using a tunable slopeenhancer for improved selectivity at the low end of the WiFi band at2400 MHz; and the low side of the low frequency band of RF filter 402used to extract cluster D may be implemented using a tunable slopeenhancer for improved selectivity on the UL channels of bands 3 and 9.

In various embodiments, the frequencies of frequency bands D and Fh arefurther filtered using band stop filters 330 and 322 and bypass switches332 and 324 as described above with respect to the embodiment of FIG.3C. However, in the embodiment of FIG. 4B, a tunable band stop filter isnot applied to cluster Fl.

FIG. 4C illustrates a frequency plan according to a further embodiment.The frequency plan of FIG. 4C is similar to the frequency plan of FIG.4A, with the exception that steep rolloff portions 304, 420 and 422 areomitted at the low end of band E, low end of band Fl and the high end ofband Fl, respectively.

FIG. 4D illustrates a conventional RF front-end 440 that can be used toimplement the frequency plan of FIG. 4C. As shown, conventional RFfront-end 440 includes separate selectable filters that are eachindividually devoted to a single channel 2, 3, 34, 38, 40 and 7; aduplexer devoted to channels 39, 41 and 38; and a filter devoted tochannels 1 and 4. As can be seen, there is redundancy in filters usedfor channel 38.

FIG. 4E illustrates embodiment RF front-end 450 that can be used toimplement the frequency plan of FIG. 4C using a smaller antenna switchand a lower component count than the conventional implementation of FIG.4D. RF front-end 450 includes an antenna switch 452 that selectivelycouples antenna 340 to fixed bandpass filters 454 and 463, to tunablebandpass filters 455 and 456 and to a signal path that does not includean acoustic filter. Bandpass filter 454 is used to select bands 1 and 4(see FIG. 3B) in cluster E, and bandpass filter 463 is used to selectband 40 in cluster Fl. The combination of tunable bandpass filter 455and tunable band reject filter 458 is used to select cluster Fhincluding bands 41, 38 and 7; and the combination of tunable bandpassfilter 456 and tunable band reject filter 460 is used to select clusterD including bands 2, 3, 34 and 39. The signal path designated as “TDDSawless” remains unfiltered by acoustic filters and is used to supportTDD operation on bands 38, 39, 40 and 41.

Bandpass filters 454 and 463, tunable bandpass filters 455 and 456, andtunable band stop filters 458 and 460 may be implemented, for example,using bandpass, tunable bandpass and tunable band stop filter structuresknown in the art and/or using tunable bandpass and band stop filterstructures disclosed herein. For example, filters 458, 460, as well asfilters 322 and 330 described above could be implemented using thefilter structures shown in FIGS. 7D, 7E, 7F, 8D, 8E, 10B, 10C and 10Dherein. Filters 455 and 456 may implemented, for example, using thefilter structures shown in FIGS. 10E, 12C, 12D, 12E and 13C herein. Theportions of filter 402 coupled between ports 1 and 4, and coupledbetween ports 1 and 2 having controllable slopes adjacent to the low endof the passband can be implemented, for example, using the tunable slopefilter shown in FIG. 10F. The portion of filter 402 coupled betweenports 1 and 3 having controllable slopes adjacent to both the low endand the high end of the passband can be implemented, for example, usingthe tunable slope filter shown in FIG. 10G.

Advantages of embodiments of the present invention ability to implementa multi-band RF-front end using wide band filters. Such embodimentsprovide area, board space and component count savings compared tosystems that use switchable narrow band filters. An additional advantageis increase design flexibility and the ability to target a same designto different frequency plans.

Notch Filter Embodiments

FIGS. 5A and 5B illustrate further conventional front end architectures.FIG. 5A illustrates a conventional RF front-end that includes parallelhigh-selectivity bandpass filters 502 that are selectable via switches504 and 506. These high-selectivity bandpass filters 502 provide a bandfiltered signal to LNA 508 and RF transceiver 510. In the illustratedexample, the noise figure of the system is degraded by the insertionloss of switches 504 and 506 and filters 502.

FIG. 5B illustrates another conventional RF front-end circuit thatincludes selectable circuit branches that each includes a lowerselectivity bandpass pre-filter 520, an LNA 524 and a bandpasspost-filter 522. Each of these circuit branches are selectable viaswitches 504 and 506. In the example of FIG. 5B, the noise figure of thesystem is degraded by the insertion loss of switches 504 and filter 520.Because the filter selectivity is split between bandpass pre-filter 520and bandpass post-filter 522, the insertion loss of filter 520 can bemade to be less than the insertion loss of bandpass filter 502 shown inFIG. 5A. Accordingly, the noise performance of the system of FIG. 5B canbe improved with respect to the system of FIG. 5A. However, the numberof filter components and LNA components is higher in the implementationof FIG. 5B.

In embodiments of the present invention, a lower selectivity bandpassfilter which may have an adjustable center frequency and a tunable bandreject filter is cascaded with an LNA. Because the tunable band rejectfilter is used to attenuate strong interferers, the lower selectivitybandpass filter may include a filter having relaxed stop bandattenuation requirements, including the portion of the stopband thatincludes the interferer (such as the transmit frequency in the case ofFDD systems). The ability to use a lower selectivity bandpass filterallows for the use of lower order filter structures that are lesscomplex and have a smaller number of filter/resonator stages. Theselower order filter structures also have less passband insertion loss,which leads to better noise performance. In one specific example, alower selectivity bandpass filter may be implemented, for example, witha ladder-type filter of the order 2.5 instead of a higher order filtersuch as a 4.5 order filter. For example, in some embodiments, the orderof the filter is 3^(rd) order or lower. Alternatively, other filterorders may be used.

In some embodiments, the lower selectivity bandpass filter may beconfigured to pass a plurality of RF bands, and the tunable band rejectfilter may be configured to reject bands that contain interferers suchas transmit signals that are transmitted by the system in an FDD mode ofoperation. In such embodiments, the size, number and complexity of thefilter components may be reduced with respect to systems that useparallel high-selectivity bandpass filters while maintaining good noiseperformance. In some embodiments, the number of RF switching componentsmay be reduced, or RF switching components may be eliminated entirelydepending on the particular embodiment.

In some embodiments, band selection filter requirements are relaxed bysuppressing a strong known but variable interferer (e.g. the own TXsignal in FDD systems) with a tunable notch filter and by distributingthe overall filter functionality in a component in front of a Low NoiseAmplifier (LNA) and a component behind the LNA. Splitting the filterinto two sections allows reducing the selectivity requirements of bandselection filter component. This again allows better in-band loss andthus better overall system noise performance. While the component beforethe LNA has direct impact on the system noise figure, the insertion lossbehind the LNA has less of an effect on the system noise figure. Using atunable band reject filter, such as a notch filter, eliminates the needfor individual filters for each band and additional LNAs and/or switchesin some embodiments.

The reduced selectivity requirements for the first filter also enablethe use of tunable bandpass filters in non-carrier-aggregation(“narrowband”) applications in which tunable bandpass filters usuallyhave less selectivity than fixed band filters. If the interferencescenario is such that one dominating interferer with a known frequency(e.g. the own TX in FDD systems) is much stronger than all otherclose-in interferes, the bandpass and band stop filters are reversed,such that the band reject filter is coupled to the input of the LNA andthe bandpass filter is coupled to the output of the LNA.

FIGS. 6A to 6G illustrate example embodiments that utilize an adjustablelower selectivity bandpass filter and a tunable band reject filter incombination with an LNA. FIG. 6A illustrates an embodiment RF front-end600 that includes bandpass filters 606 that are selectable via RFswitches 604 and 608. As shown, RF switch 604 is coupled to antenna 602.In alternative embodiments, RF switch 604 may be coupled to a differenttype of RF source such as a conductive line or a waveguide. In variousembodiments, each bandpass filter 606 has a different center frequencyand is configured to pass a plurality of RF bands present at the input.In one example embodiment, bandpass filters 606 are implemented usingfilter structures known in the art, such as SAW filters, BAW filters,FBAR filters, loosely coupled resonators, LC filters, LC resonators,tunable LC filters, microstrip filters or other filter structures withlow insertion loss at the cost of limited stop band attenuation (e.g.low filter order). As shown, RF switch 608 is coupled to LNA 610followed by tunable band reject filter 612 and RF transceiver 614. Invarious embodiments tunable band reject filter 612 may be implementedusing tunable band reject filter structures known in the art, such astunable notch filters, and/or may be implemented using tunable acousticfilter structures disclosed in U.S. Provisional Application Nos.62/595,898 and 62/641,664, as well as co-pending U.S. patent applicationSer. No. ______ corresponding to docket numbers INF-2018-P-51523USentitled, “Acoustically Coupled Resonator Notch and Bandpass Filters,”INF-2018-P-51524US entitled, “Acoustically Coupled Resonator Notch andBandpass Filter,” INF-2018-P-51527US entitled, “Tunable ResonatorElement, Filter Circuit and Method,” INF-2018-P-51528US entitled,“Tunable Resonator Element, Filter Circuit and Method,” andINF-2018-P-5529US entitled, “Acoustically Coupled Resonator Notch andBandpass Filters.” RF front-end 600 may be used, for example, in systemsthat have a strong interferer, potentially many out-of-band interferers,and/or interferers of unknown frequencies.

FIG. 6B illustrates an embodiment RF front-end 620 in which LNA 610 ispreceded by adjustable band reject/notch filter 612 and followed by aselectable bandpass filter that includes bandpass filters 622 selectablevia RF switches 604 and 608. During operation adjustable bandreject/notch filter 612 removes an interfering frequency, and bandpassfilters 622 provide further filtering at the output of LNA 610. RFfront-end 620 may be used, for example, in systems that have onedominating interferer with a known frequency and a limitation bandwidthand/or weaker interferers at frequencies that are relatively far fromthe received bands. One example of such a system is a system thattransmits at a predetermined frequency at the same time that it isreceiving at other frequencies, such as in FDD systems. RF front-end 620is also suitable for systems having a relatively large receive bandwidthand/or systems having non-contiguous spectrum blocks, as is the casewith non-contiguous carrier aggregation systems.

FIG. 6C illustrates an embodiment RF front-end 630 that is similar to RFfront-end 620 illustrated in FIG. 6B, with the exception that LNA 632 isimplemented as a bandpass LNA (BP-LNA) with integratedtunable/switchable BP behavior (e.g. matching) for improvedin-Band-performance/far-off selectivity. LNA 632 may be implemented, forexample, by using an LNA with a tunable or switchable input matchingand/or output matching and/or frequency selective gain through frequencyselective internal feedback (e.g. resonators). For example, FIG. 6Hillustrates a BP-LNA that includes LNA 610 with a tunable series inputimpedance/matching network 652; FIG. 6I illustrates a BP-LNA thatincludes LNA 610 with a tunable series output impedance/matching network654; FIG. 6J illustrates a BP-LNA that includes LNA 610 with a tunableshunt input impedance/matching network 656; and FIG. 6K illustrates aBP-LNA that includes LNA 610 with a tunable shunt inputimpedance/matching network 658. Tunable impedance/matching networks 652,654, 656 and 658 may be implemented using tunable matching networksknown in the art, such as LC and/or resonator circuits having at leastone tunable, selectable and/or switchable reactive circuit element.

FIG. 6L illustrates a BP-LNA that includes LNA 610 with a tunablebandpass filter 660 coupled between the output of LNA 610 and the inputof LNA 610. Tunable bandpass filter 660 may be implemented using tunablebandpass filter structures known in the art including LC and/orresonator circuits having at least one tunable, selectable and/orswitchable reactive circuit element or using tunable bandpass filterstructures known in the art. In alternative embodiments, a BP-LNA may beimplemented using a combination input matching networks, output matchingnetworks and tunable filters shown in FIGS. 6H-6K.

FIG. 6D illustrates an embodiment RF front-end 640 that includes tunablebandpass filter 642 followed by LNA 610. Tunable band reject/notchfilter 612 is coupled to the output to LNA 610 and may be used to rejectstrong interferers. In various embodiments, tunable bandpass filter 642is implemented using tunable bandpass filter structures known in the artor may be implemented using tunable acoustic filter structures disclosedin U.S. Provisional Application Nos. 62/595,898 and 62/641,664, as wellas co-pending U.S. patent application Ser. No. ______ corresponding todocket numbers INF-2018-P-51523US entitled, “Acoustically CoupledResonator Notch and Bandpass Filters,” INF-2018-P-51524US entitled,“Acoustically Coupled Resonator Notch and Bandpass Filter,”INF-2018-P-51527US entitled, “Tunable Resonator Element, Filter Circuitand Method,” INF-2018-P-51528US entitled, “Tunable Resonator Element,Filter Circuit and Method,” and INF-2018-P-51529US entitled,“Acoustically Coupled Resonator Notch and Bandpass Filters.” In someembodiments, tunable bandpass filter 642 is a continuously tunablebandpass filter. RF front-end 640 is suitable for systems having arelatively low receive bandwidth, including, but not limited to singlecarrier LTE, UMTS, narrow-BW LTE, multiband internet of things (IOT),and multiband wearables/watches. RF front-end 640 is also suitable forsystems having one dominating interferer with a known frequency and alimited bandwidth, such as an FDD system that transmits at frequenciesclose to a receive frequency. In some embodiments, band reject/notchfilter 612 coupled to the output to LNA 610 may be omitted.

FIG. 6E illustrates an embodiment RF front-end 650 that includes tunableband reject/notch filter 612 followed by LNA 610. Tunable bandreject/notch filter 612 may be used, for example, to suppress dominatinginterferers with good in-band insertion loss. Tunable bandpass filter642 is coupled to the output to LNA 610. System 650 is suitable forsystems having a relatively low receive bandwidth, including, but notlimited to single carrier LTE, UMTS, narrow-BW LTE, multiband internetof things (IOT), and multiband wearables/watches. RF front-end 650 isalso suitable for systems having one dominating interferer with a knownfrequency and a limited bandwidth, such as an FDD system that transmitsat frequencies close to a receive frequency. In some embodiments,tunable bandpass filter 642 coupled to the output of LNA 610 may beomitted.

FIG. 6F illustrates an embodiment RF front-end 660 that includes tunablebandpass filter 642 followed by tunable BP-LNA 632. Tunable band rejectfilter 612 is coupled to the output of tunable BP-LNA 632 and may beused to reject strong interferers. RF front-end 660 is similar to RFfront-end 640 shown in FIG. 6D with the exception that LNA 610 isreplaced by BP-LNA 632, which provides better far-off selectivity insome embodiments. In some embodiments, tunable band reject/notch filter612 coupled to the output of tunable BP-LNA 632 may be omitted.

FIG. 6G illustrates an embodiment RF front-end 680 that includes tunableband reject filter 612 followed by tunable BP-LNA 632. Tunable bandpassfilter 642 is coupled to the output of tunable BP-LNA 632. RF front-end680 is similar to RF front-end 650 shown in FIG. 6E with the exceptionthat LNA 610 is replaced by tunable BP-LNA 632, which provides enhancedfar-off selectivity in some embodiments. In some embodiments, tunablebandpass filter 642 coupled to the output of tunable BP-LNA 632 may beomitted.

The filters depicted in the embodiments herein can be implemented, forexample, using filter structures shown in FIGS. 7A-7N, 8A-8E, 9A-9C,10A-10D, 11A-11C and 12A-12E and 13A-13E discussed below. These filtersmay be implemented using capacitors 702, inductors 704, capacitors 705,acoustically coupled resonators 706 having two coupled resonators,tunable resonators 708, acoustically coupled resonator structure 710having a plurality of series resonators acoustically coupled to eachother, parallel resonant tuning circuits 712 having a tunable acapacitor coupled in parallel with an inductor, series parallel resonanttuning circuits 714 having a tunable a capacitor coupled in series withan inductor, resonators 716, and tunable capacitors 718. In someembodiments, these filters may be implemented using the physicalacoustic filter structures and tunable acoustic filter structuresdisclosed in U.S. Provisional Application Nos. 62/595,898 and62/641,664, as well as co-pending U.S. patent application Ser. No.______ corresponding to docket numbers INF-2018-P-51523US entitled,“Acoustically Coupled Resonator Notch and Bandpass Filters,”INF-2018-P-51524US entitled, “Acoustically Coupled Resonator Notch andBandpass Filter,” INF-2018-P-51527US entitled, “Tunable ResonatorElement, Filter Circuit and Method,” INF-2018-P-51528US entitled,“Tunable Resonator Element, Filter Circuit and Method,” andINF-2018-P-51529US entitled, “Acoustically Coupled Resonator Notch andBandpass Filters.”

FIGS. 7A-7N illustrate various bridged T all-pass circuits that can beused to implement the various band stop filters utilized in allembodiments described with respect to the embodiment circuits of FIGS.2A-2D, 3A-3N, 4A-4D. 6A-6G, 15B, 17A-17C, 18A-18D, 19A-19C, 20A-20B,21A-21B and 22.

FIG. 7A illustrates a bridged T all-pass circuit 700 that includes a “T”structure having capacitors 702 with capacitance values C1 and C2coupled in series between nodes 102, an inductor 704 having aninductance value of L1 coupled between ports 1 and 2, and an inductor704 having an inductance L2 coupled to ground. Bridged T all-passcircuit 700 has a flat amplitude response between ports 1 and 2 when thefollowing conditions are met:

$\begin{matrix}{{L\; 1} = \frac{Z_{0}}{\omega_{0}}} & (1) \\{{L\; 2} = \frac{Z_{0}}{2\omega_{0}}} & (2) \\{{{C\; 1} = {{C\; 2} = \frac{1}{Z_{0}\omega_{0}}}},} & (3)\end{matrix}$

where Z₀ is the characteristic impedance that loads ports 1 and 2 and ω₀is the radian frequency in which the phase response between ports 1 and2 reaches 90°. In various embodiments, bridged T all-pass circuit 700can be configured to have a band stop response when capacitors 702 areeach replaced by a 2-port resonators 703 as shown with respect tocircuit 710 or are both replaced by a 3-port resonator 705 as shown withrespect to circuit 720. Specific examples of such embodiments, as wellas embodiments that utilize other LC networks, or combinations of LCnetworks and resonators are illustrated in FIGS. 7C to 7G. In suchembodiments, the out of band response (outside the band stopfrequencies) maintains its original all-pass characteristics.

In a further embodiment, a band stop response may be achieved bydetuning the values for L1, L2, C1 and/or C2 from their values definedin equations (1), (2) and (3) shown above. Detuning may be used toachieve band stop responses with higher bandwidths.

FIG. 7B illustrates a bridged T all-pass circuit 730 that includes a “T”structure having inductors 704 with inductance values L1 and L2 coupledin series between nodes 102, a capacitor 702 having capacitance value ofC1 coupled between ports 1 and 2, and capacitor 702 having a capacitancevalue of C2 coupled to ground. Bridged T all-pass circuit 730 has a flatamplitude response between ports 1 and 2 when the following conditionsare met:

$\begin{matrix}{{L\; 1} = {{L\; 2} = \frac{Z_{0}}{\omega_{0}}}} & (4) \\{{C\; 1} = \frac{1}{2Z_{0}\omega_{0}}} & (5) \\{{C\; 2} = {\frac{2}{Z_{0}\omega_{0}}.}} & (6)\end{matrix}$

In various embodiments, bridged T all-pass circuit 730 can be configuredto have a band stop response when inductors 704 are each replaced by a2-port resonators 703 as shown with respect to circuit 740 or are bothreplaced by a 3-port resonator 705 as shown with respect to circuit 750.Specific examples of such embodiments, as well as embodiments thatutilize other LC networks, or combinations of LC networks and resonatorsare illustrated in FIGS. 7H to 7N. In such embodiments, the out of bandresponse (outside the band stop frequencies) maintains its originalall-pass characteristics.

In a further embodiment, a band stop response may be achieved bydetuning the values for L1, L2, C1 and/or C2 from their values definedin equations (4), (5) and (6) shown above. Detuning may be used toachieve band stop responses with higher bandwidths.

FIGS. 8A-8E illustrate various Pi Low-Pass based circuits that can beused to implement the various band stop filters utilized in allembodiments described with respect to the embodiment circuits of FIGS.2A-2D, 3A-3N, 4A-4D. 6A-6G, 15B, 17A-17C, 18A-18D, 19A-19C, 20A-20B,21A-21B and 22.

FIGS. 9A-9C illustrate various Triplet based circuits; FIGS. 10A-10Gillustrate various ladder based circuits; and FIGS. 11A-11C illustratevarious lattice based circuits that can be used to implement the variousband stop and bandpass filters utilized in all embodiments describedwith respect to the circuits of FIGS. 2A-2D, 3A-3N, 4A-4D, 6A-6G,15A-15B, 17A-17C, 18A-18D, 19A-19C, 20A-20B, 21A-21B, 22, 23, 24A, 25,26A and 27.

FIGS. 12A-12E illustrate various CRF based circuits; and FIGS. 13A-13Eillustrate various Pi Low-Pass based circuits that can be used toimplement the various bandpass filters utilized in all embodimentsdescribed with respect to the embodiment circuits of FIGS. 2A-2D, 3A-3N,4A-4D, 6A-6G, 15A, 17A-17C, 21A-21B, 22, 23, 24A, 25, 26A and 27.

As shown, the embodiment filter structures of FIGS. 7A-7F, 8A-8E, 9A-9C,10A-10D, 11A-11C and 12A-12E and 13A-13E, can be implemented using avariety of LC filter structures, acoustic filter structures, and tunableLC filter/acoustic filter structures.

FIG. 14 illustrates a block diagram of an example system 1400 in apackage according to an embodiment of the present invention. As shown,system 1400 includes antenna switch 1402 that is coupled to a pluralityof RF signal paths having respective outputs RF_OUT1, RF_OUT1 andRF_OUT3. The first RF path includes a fixed bandpass filter 1404followed by LNA 1416; the second RF path includes tunable bandpassfilter 1406 and tunable band stop/notch filter 1408 followed by LNA1418; and the third RF path includes LNA 1420 followed by tunable bandstop/notch filter 1414. These RF signal paths may be implemented usingembodiment circuits, methods and physical implementations describedherein. In an embodiment, tuning actuators 1410 are used to providetuning and control signals to tunable bandpass filter 1406 and totunable band stop/notch filters 1408 and 1414. LNA bias generator 1412is used to provide bias currents and voltages to LNAs 1416, 1418 and1420. In some embodiments, antenna switch 1402, tuning actuators 1410and LNA bias generator 1412 are controllable via a digital bus, such asa MIPI bus, via digital interface/control circuit 1422. Alternatively,other digital interface types may be used. It should be understood thatthe configuration of system 1400 is just one example of many possiblesystem implementations. In alternative embodiments, a different numberof RF signal paths and/or different embodiment filter configurations maybe implemented. In various embodiments, system 1404 may be implementedas multiple components on a package substrate, or on a single monolithicsemiconductor substrate.

Advantages of embodiments include the ability to implement a flexibleand physically compact RF front end that is able to withstand stronginterferers, such as high amplitude transmit signals generated by the RFsystem when operating in an FDD mode. By using a tunable band rejectfilter in conjunction with a lower selectivity tunable bandpass filter,extra size overhead due to the increased size and number of higher orderRF filters can be reduced. In addition, the use of tunable filtersallows for the ability to tune the system to a variety of RF channelsand the ability to reject interferers in a variety of RF environmentswithout the need for system redesign in some embodiments.

FIGS. 15A and 15B illustrate conventional embodiments ofmulti-band/channel multi-transceiver front end circuits that might befound, for example, in Cellular Phones, Smart Watches, Wearables, IoT(Internet-of-Things) devices. In such devices, multiple transmitters mayoperate simultaneously using Uplink Carrier Aggregation (ULCA) or DualConnectivity. During operation, the amount of power transmitted by eachtransmitter is controlled in a closed-loop fashion by measuring thepower transmitted by the device and adjusting the transmitted power tomeet a target using a feedback loop. This feedback loop might be used tomeasure the quality of the transmitted output signal (for e.g. adaptivedigital pre-distortion) or for antenna impedance measurements (for e.g.closed loop antenna tuning). Both techniques may be used to optimizepower consumption and improve output signal performance.

However, in systems having more than one active transmitter,interference signals may combine with the transmit signal at the pointwhere a particular transmit signal is to be measured for a particulartransmitter. These interference signals may be generated, for example,by other transmitters in the system, and/or may be generated by mixingof various signals present in the system that creates mixing productsthat fall within the bandwidth of the transmit power measurementcircuit. These interferer signals can lead to degradation of theaccuracy of the feedback path.

FIG. 15A illustrates a conventional RF front end 1500 that includes twoRF transceiver circuits 1502 and 1504. Each transceiver circuit 1502 and1504 includes a single transmit output and four receiver inputs. Thetransmit output of each RF transceiver circuit 1502 and 1504 is coupledto a power amplifier 1510 followed by selection switch 1512 and duplexercircuits 1516. Selection switch 1518 selects a duplexer 1516 from amongdiplexers 1516 to be coupled to antenna 1526 or 1527 via directionalcoupler 1522 and tunable antenna matching circuit 1524. Duplexers 1516may have different passbands and may be configured to provide channelselectivity in the receive direction and transmit filtering in thetransmit direction. LNAs 1514 have respective inputs coupled tocorresponding duplexers 1516 and outputs coupled to respective channelsof transceiver 1502 or 1504.

During operation, transceiver 1502 and 1504 measure the power, signalquality and/or other parameters of the signal transmitted by PA 1510 bymeasuring a coupled output of directional coupler 1522. Other measuredparameters of the signal transmitted by PA 1510 may include, forexample, signal phases and amplitudes used to determine antennaimpedances and/or signal quality in adaptive pre-distortion systems.Switch 1520 may be used to select a coupler output that provides coupledincident power and coupled reflected power. Lowpass filter 1530 is usedto filter coupled power provided by directional coupler 1522. In somecircumstances, the frequency range of interfering signals may be withinthe passband of lowpass filter 1530. Such circumstances may arise, forexample when RF transceivers 1502 and 1504 simultaneously transmitsignals and the RF signal produced by one RF transceiver 1502 or 1504 iswithin the pass band of the lowpass filter 1530 associated with theother RF transceiver 1502 or 1504. Issues may also arise when thefundamental frequency of one RF transceiver 1502 or 1504 is not withinthe passband of the lowpass filter 1530 of the other RF transceiver 1502or 1504. For example, when a distortion and/or intermodulation productof one RF transceiver may be within the passband of lowpass filter 1530of the other RF transceiver. This situation may be exacerbated, forexample, in situations where one RF transceiver 1502 or 1504 transmitsat a much higher amplitude than the other transceiver 1502 or 1504. Insuch circumstances, the leaking TX signal and/or distortion productsproduced in one transmit channel may have a power that is on the sameorder of the transmit power produced by the other transmit channels atthe TX feedback receiver input.

FIG. 15B illustrates a conventional RF front end 1550 that includes RFtransceiver circuits 1502 and 1504. RF front end 1550 has a similarstructure as RF front end 1500 of FIG. 15A with the exception that theeach filtered RF channel has its own directional coupler 1522 associatedwith it. As shown, a directional coupler 1522 is coupled in series witheach duplexer circuit 1516, and the outputs of all duplexer circuits1516 are selectable via selection switch 1552. In RF front end 1550, thefiltering provided by each duplexer 1516 attenuates transmitted signalsproduced by the other transmitters and provides better transmit signalaccuracy over RF system 1500 shown in FIG. 15A. However, RF system 1550has an increased component count and system complexity. As shown, RFsystem 1500 has four times as many directional couplers 155 and twoadditional RF selection switches 1552. Another issue with thearchitecture shown in FIG. 15B is that having the measurement planedefined by the coupler position “in front of” the duplexers does notinclude the duplexer insertion loss in (antenna) power measurements andmakes other measurements, such as antenna impedance measurements, morecomplicated to perform.

FIG. 16A illustrates an RF system 1600 according to an embodiment of thepresent invention. The architecture of RF system 1600 is similar to thatof RF system 1500 shown in FIG. 15A, with the exception that lowpassfilter 1530 coupled to the transmit feedback input of RF transceivers1502 and 1504 are replaced by tunable bandpass filters 1602. By usingtunable bandpass filters 1602, interference signals produced within thesystem can be more effectively filtered from the transmit signal (e.g.power, signal quality and antenna impedance measurements). In addition,the more accurate transmit power measurements may be made without usinga separate directional coupler and associated RF switches for eachdiplexer 1516 in some embodiments.

During operation, the center frequency of tunable bandpass filters 1602may be tuned to have a center frequency that corresponds with the centerfrequency of the transmit signal and/or a passband that includes thetransmitted frequency. Feedback receiver 1603 of RF transceiver 1502 or1504 measures one or more filtered coupled signal parameters (e.g.power, phase, signal quality, error vector magnitude (EVM), linear andnon-linear distortion) of the signal output from bandpass filter 1602.This measured power may be used, for example, to determine, adjust orupdate the power of the transmit signal provided to power amplifier1610. In embodiments directed to feedback receivers, these measurementsmay be used to implement antenna impedance measurements or transmitsignal quality measurements for adaptive transmit signal predistortion.In some embodiments, each RF transceiver circuit 1502 or 1504 producesthe tuning signal for its associated tunable bandpass filter 1602. Insome embodiments, the tuning signals for the tunable bandpass filters1602 are produced by a central controller.

In various embodiments, tunable bandpass filters 1602 are eachimplemented using tunable bandpass filter structures known in the art ormay be implemented using tunable acoustic filter structures disclosed inU.S. Provisional Application Nos. 62/595,898 and 62/641,664, as well asco-pending U.S. patent application Ser. No. ______ corresponding todocket numbers INF-2018-P-51523US entitled, “Acoustically CoupledResonator Notch and Bandpass Filters,” INF-2018-P-51524US entitled,“Acoustically Coupled Resonator Notch and Bandpass Filter,”INF-2018-P-51527US entitled, “Tunable Resonator Element, Filter Circuitand Method,” INF-2018-P-51528US entitled, “Tunable Resonator Element,Filter Circuit and Method,” and INF-2018-P-51529US entitled,“Acoustically Coupled Resonator Notch and Bandpass Filters.” In someembodiments, tunable bandpass filter 1602 is a continuously tunablebandpass filter. Tunable bandpass filter 1602 may be implemented usingthe tunable acoustic filter based bandpass structures illustrated inFIGS. 9A-9C, 10A-10G, 11A-11C, 12A-12E and 13A-13E. In some embodiments,embodiment bandpass filter 1602 may have an order of 3.5 and greater andmay contain 5-7 resonators. Alternatively, lower order filters orfilters having greater or fewer than 5-7 resonators may be used.

FIG. 16B illustrates an RF system 1600 according to a further embodimentof the present invention. The architecture of RF system 1600 is similarto that of RF system 1500 shown in FIG. 15A, with the exception thatlowpass filter 1530 coupled to the transmit feedback input of RFtransceivers 1502 and 1504 are replaced by tunable band stop filters1612. By using tunable band stop filters 1602, interference signalsproduced within the system can be selectively removed from the transmitsignal measurements. In addition, more accurate transmit signalmeasurements may be made without using a separate directional couplerand associated RF switches for each diplexer 1516. In some embodiments,two or more tunable band stop filters 1612 may be cascaded in order toreject signals at multiple frequencies.

During operation, the center frequency of tunable band stop filters 1602may be turned to have a center frequency that corresponds with thecenter frequency of the transmit signal and/or a center frequency thatcorresponds to a frequency of a known or anticipated interferer.Feedback receiver 1603 of RF transceiver 1502 or 1504 measures thefiltered coupled signal parameters (e.g. power, phase, signal quality)output from bandpass filter 1602. This measured power may be used, forexample, to determine, adjust or update the power of the transmit signalprovided to power amplifier 1610. In some embodiments each RFtransceiver circuit 1502 or 1504 produces the tuning signal for itsassociated tunable band stop filter 1612. In such embodiments each RFtransceiver circuit 1502 or 1504 has knowledge of the frequency overwhich the other RF transceiver circuit 1502 or 1504 is transmitting. Inother embodiments, the RF transceiver circuit 1502 produces the tuningsignal associated with the band stop filter 1612 associated with RFtransceiver circuit 1504 and vice versa. In some embodiments, tuningsignals for the tunable band stop filters 1612 are produced by a centralcontroller.

In various embodiments, tunable band stop filters 1612 are eachimplemented using tunable band stop filter structures known in the artor may be implemented using tunable acoustic filter structures disclosedin U.S. Provisional Application Nos. 62/595,898 and 62/641,664, as wellas co-pending U.S. patent application Ser. No. ______ corresponding todocket numbers INF-2018-P-51523US entitled, “Acoustically CoupledResonator Notch and Bandpass Filters,” INF-2018-P-51524US entitled,“Acoustically Coupled Resonator Notch and Bandpass Filter,”INF-2018-P-51527US entitled, “Tunable Resonator Element, Filter Circuitand Method,” INF-2018-P-51528US entitled, “Tunable Resonator Element,Filter Circuit and Method,” and INF-2018-P-51529US entitled,“Acoustically Coupled Resonator Notch and Bandpass Filters.” In someembodiments, tunable band stop filter 1612 is a continuously tunableband stop filter. Tunable band stop filter 1612 may be implemented usingthe tunable acoustic filter structures illustrated in FIGS. 7A-7F,8A-8E, 9A-9C, and 11A-11C. In some embodiments, embodiment bandpassfilter 1602 may start with an order of 1 using, for example, tworesonators. In one specific example, an order of 3.5 using sevenresonators is used. It should be appreciated, however, that any order ornumber of resonators may be used depending on the particular systembeing implemented and its specifications.

It should be understood that the embodiments of FIGS. 16A and 16B arejust two specific examples of embodiments multi-channel front endcircuits. In alternative embodiments of the present invention, greateror fewer than four receive paths may be implemented for the receivechannels of RF transceivers 1502 and/or 1504, and greater than onetransmit channel may be provided for the transmit channels of RFtransceivers 1502 and/or 1504. Moreover different receive and transmittopologies could be used depending on the particular system beingimplemented and it specifications. Besides power detection, otherfunctions can be performed such as measuring TX linearity of the TXoutput signal (for adaptation of digital predistortion or envelopetracking) or antenna impedance measurement for antenna tuning (measuringamplitude and phase of forward and reflected signals). For simplicity,transceivers 1502 and 1504 are shown as separate devices. However, theycan be integrated into one physical device in an embodiment.

Just as embodiments of the present invention can be applied to thereceive path of an RF transceiver, embodiments of the present inventioncan also be applied in a similar way to the transmit path of an RFtransceiver as described herein with respect to FIGS. 17A-C, 18A-D,19A-C, 20A-B and 21A-B. Such embodiments can be directed, for example,to systems such as cellular phones, smart watches, wearables, and IoT(Internet-of-Things) devices. More specifically, embodiments may bedirected to multi-band RF frontend designs for systems including but notlimited to FDD systems, systems that utilize uplink carrier aggregation,and dual/multi-connect systems in which multiple transmitters are activeat the same time.

The growing complexity of RF frontends due to the growing number ofsupported bands, modes, and multiple connections results in higherinsertion loss in practical systems implementations. This higherinsertion loss leads to increased power amplifier current consumption inthe transmit path and reduced receiver reference sensitivity. Systemsthat have multiple transmitters active at the same time, for example tosupport uplink carrier aggregation, and dual/multi-Connect Systems(Multi-Sim, Multi-RAT, cellular/Wi-Fi) may experience intermodulationbetween the different transmit signals resulting in difficulty infulfilling spectral emission and self-interference requirements and thatimpact receiver performance.

Current FDD (Frequency Division Duplex) solutions may use high isolationduplex filters to provide high isolation between transmit and receivepaths at the receive frequency band to minimize the directdesensitization of the receiver by noise produced in the transmitter,and at the transmit frequency band to protect the sensitive receiverfrom high power transmit signals that may degrade the performance of thereceiver due to non-linear effects such as IP2, IP3, and reciprocalmixing. The use of these high isolation duplex filters, however, comesat a cost of duplex filter complexity and insertion loss.

In embodiments of the present invention, one or more reconfigurable(e.g. tunable or switchable) band stop filters in the transmit path ofan RF system is used to improve overall system performance with respectto one or more of radio front-end complexity, PCB area, transmit powerconsumption, transmit spectral emission purity, and receive referencesensitivity. Such improvement is addressed using a variety of techniquesas described below.

Advantages of embodiments of the present invention include the abilityto support multiple transmit bands, transmit mode and multipleconnections with decreased power amplifier current consumption andincreased reference sensitivity.

FIGS. 17A-17C illustrate block diagram of embodiment RF systems thatinclude a combined receive/transmit antenna and a duplexer. Arequirement of a duplexer is to provide isolation between the TX pathand RX in the transmit frequency band as well as in the receivefrequency band. The isolation requirements in the receive frequency bandof the duplexer are relaxed by using band stop filters in the transmitpath in front of and/or after the power amplifier (PA) to attenuateenergy within the receive frequency band. In front of the PA a filtercan also be placed at the image frequency band of the receive frequencyband, which would also be folded to the receive frequency band bynon-linear effects in the PA. Optionally, the isolation requirements inthe transmit frequency band of the duplexer can be relaxed by usingtunable filter (which can be a band stop, bandpass, or other filter) infront of and/or after the low noise amplifier (LNA) in the receive path.In some embodiments, this may result in a relaxation of about 7 dB ofisolation requirements from about 45 dB to 38 dB in one example. Byrelaxing these isolation requirements, a lower order duplexer with lowerinsertion loss and potentially smaller package size may be used in thesystem.

FIG. 17A illustrates an embodiment RF system 1700 that includes an RFtransceiver 1702 that has a transmit output (TX) and a receive input(RX). The transmit path includes a tunable band stop filter 1710,followed by power amplifier (PA) 1704, duplexer 1716 and antenna 1708.In various embodiments, tunable band stop filter 1710 is tuned to have anotch at the receive frequency. The receive path includes LNA 1706,tunable filter 1714 coupled to the input of LNA 1706 and tunable filter1712 coupled to the output of LNA 1706. These tunable filters 1712 and1714 may configured to be band stop filters that are tuned to thetransmit frequency and/or may be configured to be bandpass filters thatare tuned to the receive frequency. The use of filters 1712 and 1714 maybe used to further reduce the isolation requirements of duplexer filter1716. It should be understood that the inclusion of one or both oftunable filters 1712 and 1714 is optional and some embodiment systemsomit these filters.

As shown in FIG. 17A, band stop filter 1710 is coupled to the input ofpower amplifier 1704. By coupling tunable band stop filter 1710 to theinput of power amplifier 1704, which is in a lower power domain than theoutput of power amplifier 1704, the insertion loss of tunable band stopfilter 1710 has a negligible effect on the power consumption of thesystem.

In some embodiments, power amplifier 1704 includes a plurality of poweramplifier stages coupled in series. In some embodiments, the pluralityof power amplifier stages may have additional filtering between eachstage, such as additional band stop filtering. Alternatively, in someembodiments, no additional band stop filtering is provided between eachstage of power amplifier 1704.

FIG. 17B illustrates an embodiment RF system 1720 that is similar to RFsystem 1700 of FIG. 17A, but further includes a second tunable band stopfilter 1722 coupled in series with tunable band stop filter 1710 at theinput of power amplifier 1704. In an embodiment, tunable band stopfilter 1722 is tuned to the receive image frequency. Thus, any energy atthe receive image frequency that present at the transmit output of RFtransceiver 1702 gets attenuated prior to its being mixed to the receivefrequency. In some embodiments, this may further relax the isolationrequirements of duplexer 1716.

FIG. 17C illustrates an embodiment RF system 1730 that is similar to RFsystem 1720 of FIG. 17B, but further includes a third tunable band stopfilter 1732 coupled to the output of power amplifier 1704. In variousembodiments, tunable band stop filter 1732 is tuned to the receivefrequency. In some embodiments, tunable band stop filters 1710 and 1722at the input to power amplifier 1704 may be omitted.

FIGS. 18A-18D illustrate embodiment RF systems having a combinedreceive/transmit antenna in which the duplexer is replaced with tunableband stop filters and adjustable phase shifters/matching networks in thetransmit path and in the receive path. In various embodiments, a tunableband stop filter in the transmit path is tuned to the receive frequencyin order to attenuate transmit noise at the receive frequency, and anadjustable phase shifter/matching network in the transmit path isadjusted to transform the impedance of the adjustable band stop filterat the notched receive frequency so that the impedance approximates anopen circuit and does not load the output at the receive frequency.Similarly, a tunable band stop filter in the receive path is tuned tothe transmit frequency to attenuate the transmit signal before it hitsthe receiver, and an adjustable phase shifter/matching network in thereceive path is adjusted to transform the impedance of the adjustableband stop filter at the notched transmit frequency so that the impedanceapproximates an open circuit and does not load the output at thetransmit frequency. In some embodiments the impedance transformationperformed by the adjustable phase shifter/matching networks describedherein transforms an impedance from a lower impedance to a higherimpedance.

Advantages of such embodiments include the ability eliminate a fixedfrequency duplexer in an RF system that operates at different frequencybands, such as different LTE bands. This way one tunabletransmit/receive path pair can cover multiple bands and replace multipletransmit/receive path pairs with fixed frequency duplexers.

FIG. 18A illustrates an embodiment RF system 1800 that includes an RFtransceiver 1702 having a transmit output (TX) and a receive output(RX). The transmit path includes power amplifier 1704 followed bytunable band stop filter 1802, tunable phase shifter 1804 and antenna1708. The receive path includes tunable phase shifter 1808 followedtunable band stop filter 1806 and LNA 1706, the output of which iscoupled to the receive input of RF transceiver 1702.

In an embodiment, band stop filter 1802 of the transmit path is tuned tothe receive frequency in order to attenuate noise generated by thetransmit path at the receive frequency and reduce the amount of noisereceived at the receive frequency. Phase shifter 1804 turned such thatthe output impedance of tunable band stop filter 1802 at the notchfrequency is transformed to an impedance approximating an open circuitat the output of phase shifter 1804. This prevents tunable band stopfilter 1802 from loading the output of RF system 1800 at the receivefrequency.

Similarly, band stop filter 1806 of the receive path is tuned to thetransmit frequency in order to attenuate the transmit signal generatedby RF transceiver 1702 and prevent the transmit signal from overloadingand/or desensitizing the LNA. Phase shifter 1808 is tuned such that theinput impedance of tunable band stop filter 1806 at the notch frequencyis transformed to an impedance approximating an open circuit at input ofphase shifter 1808 (e.g. the port of phase shifter 1808 coupled toantenna 1708). This prevents tunable band stop filter 1806 from loadingthe output of RF system 1800 at the transmit frequency.

FIG. 18B illustrates an embodiment RF system 1820 that is similar to RFsystem 1800 of FIG. 18A, but further includes a further tunable bandstop filter 1822 coupled between the output of the transmit output of RFtransceiver 1702 and the input of power amplifier 1704. In anembodiment, tunable band stop filter 1822 is also tuned to the receivefrequency in order to further attenuate energy generated by thetransmitter of RF transceiver 1702.

FIG. 18C illustrates an embodiment RF system 1830 that is similar to RFsystem 1820 of FIG. 18B, but further includes another tunable band stopfilter 1832 in addition to tunable band stop filter 1822 coupled betweenthe output of the transmit output of RF transceiver 1702 and the inputof power amplifier 1704. In an embodiment, tunable band stop filter 1832is tuned to the receive image frequency. Thus, any energy at the receiveimage frequency that present at the transmit output of RF transceiver1702 get attenuated prior to it being mixed to the receive frequency bynonlinearities in power amplifier 1704.

FIG. 18D illustrates an embodiment RF system 1840 that is similar to RFsystem 1830 of FIG. 18C with the further addition of tunable band stopfilter 1842 coupled between the output of LNA 1706 and the receive inputof RF transceiver 1702. In an embodiment, tunable band stop filter 1842is tuned to the transmit frequency to provide further transmitisolation. It should be understood that FIGS. 18A-18D are just fourexample embodiments. Other RF systems and configurations that use bandstop filters and phase shifters instead duplex filters may be possible.For example, tunable band stop filter 1842 may also be coupled betweenthe LNA 1706 and the receive input of RF transceiver 1702 in theembodiments of FIGS. 18A and 18B.

FIGS. 19A-19C illustrate embodiment RF systems in which isolationbetween the transmit path and the receive path is achieved by usingseparate transmit and receive antennas instead of using a duplex filterand a single antenna. In addition, tunable band stop filters are used toattenuate frequencies at the receive frequency and/or the receive imagefrequency in the transmit path and to attenuate the transmit frequencywithin the receive path.

Advantages of such embodiments include the ability to replace multiplefixed frequency duplexers that would normally be needed for different(e.g. LTE) frequency bands by a single pair of configurable band stopfilters in the receive signal path and the transmit signal path.

FIG. 19A illustrates an embodiment RF system 1900 that includes an RFtransceiver 1702 having a transmit output (TX) and a receive input (RX).The transmit path includes a tunable band stop filter 1906, followed bypower amplifier (PA) 1704 and transmit antenna 1902. The receive pathincludes LNA 1706, tunable band stop filter 1908 coupled to the input ofLNA 1706 and receive antenna 1904. Tunable band stop filter 1906 in thetransmit path is tuned to the receive frequency to attenuate noisegenerated by transmit circuitry of RF transceiver 1702 that would becoupled back to the receive channel of RF transceiver 1702 via antennas1902 and 1904; and tunable band stop filter 1908 in the receive path istuned to the transmit frequency to prevent the transmit signal generatedby RF transceiver 1702 from overloading and/or desensitizing the inputof LNA 1706 and/or the receive circuitry of RF transceiver 1702.

FIG. 19B illustrates an embodiment RF system 1920 that is similar to RFsystem 1900 of FIG. 19A, but further includes a second tunable band stopfilter 1922 coupled in series with tunable band stop filter 1906 at theinput of power amplifier 1704. In an embodiment, tunable band stopfilter 1922 is tuned to the receive image frequency. Thus, any energy atthe receive image frequency that present at the transmit output of RFtransceiver 1702 get attenuated prior to its being mixed to the receivefrequency.

FIG. 19C illustrates an embodiment RF system 1930 that is similar to RFsystem 1920 of FIG. 19B, but further includes a fourth tunable band stopfilter 1932 coupled to the output of power amplifier 1704, and fifthtunable band stop filter 1934 coupled between the output of LNA 1706 andthe receive input of RF transceiver 1702. In various embodiments, bandstop filter 1932 is either tuned to the receive frequency to reduce theamount of noise that reaches LNA 1706. In some embodiments, tunable bandstop filters 1906 and 1922 at the input to power amplifier 1704 may beomitted.

FIGS. 20A-20B illustrate embodiment RF systems directed tomulti-transmitter systems that have more than one transmitter active atthe same time coupled to the same physical antenna. However, instead ofhaving a duplexer isolate the two transmit paths, band stop filters andadjustable phase shifter/matching networks are used to isolate the twotransmit signal paths. Advantages of such embodiments include theability eliminate a fixed frequency duplexer in an RF system thatsimultaneously transmits in different frequency bands, such as differentLTE bands.

FIG. 20A illustrates RF system 2000 that includes an RF transceiver 2002having two transmit outputs TX1 and TX2. A first transmit path includesa first power amplifier 2004 followed by a tunable band stop filter 2006and an adjustable phase shifter/matching network 2008, which is coupledto antenna 2010. Similarly, a second transmit path includes a secondpower amplifier 2012 followed by a tunable band stop filter 2014 and anadjustable phase shifter/matching network 2016, which is also coupledantenna 2010. The notch frequency of tunable band stop filter 2006 ofthe first transmit path is tuned to the transmit frequency of the secondsignal path, while the notch frequency of tunable band stop filter 2014of the second transmit path is tuned to the transmit frequency of thefirst signal path. By notching out the transmit frequency of the othertransmit signal path, the intermodulation distortion between the twotransmit frequencies and overloading of the output of the two poweramplifiers 2004 and 2012 is reduced and/or avoided.

Adjustable phase shifter/matching network 2008 transforms the outputimpedance of adjustable band stop filter 2006 at its notch frequency toan impedance approximating an open circuit at the interface to antenna2010 in order to avoid loading the output. Similarly, adjustable phaseshifter/matching network 2016 transforms the output impedance ofadjustable band stop filter 2014 at its notch frequency to an impedanceapproximating an open circuit at the interface to antenna 2010 in orderto avoid loading the output.

The concept of RF system 2000 shown in FIG. 20A can also be extended toincorporate more channels, or even the addition of a receive channel inaddition to the two transmit channels. For example, as shown in FIG.20B, RF system 2030 includes an RF transceiver 2032 that includes twotransmit outputs TX1 and TX2, as well as a receive input RX. The firsttransmit path coupled to the first transmit output TX1 includes poweramplifier 2034, followed by two adjustable band stop filters 2036 and2038, which are followed by adjustable phase shifter/matching network2040 coupled to shared antenna 2042. Similarly, the second transmit pathcoupled to the second transmit output TX2 includes power amplifier 2044,followed by two adjustable band stop filters 2046 and 2048, which arefollowed by adjustable phase shifter/matching network 2050 coupled tothe shared antenna 2042. A receive path includes adjustable matchingnetwork 2058 coupled to shared antenna 2042, tunable band stop filters2054 and 2056 and LNA 2052 having an output coupled to the receive inputRX of RF transceiver 2032.

Tunable band stop filters 2036 and 2038 of the first transmit path aretuned to provide a notch at the transmit frequency of the secondtransmit path and a notch at the receive frequency of the receive path.Adjustable phase shifter/matching network 2040 is configured totransform the impedances at the notch frequencies of adjustable bandstop filters 2036 and 2038 to an output impedance approximating an opencircuit to avoid loading the output at these notch frequencies.

Similarly, tunable band stop filters 2046 and 2048 of the first transmitpath are tuned to provide a notch at the transmit frequency of the firsttransmit path and a notch at the receive frequency of the receive path.Adjustable phase shifter/matching network 2050 is configured totransform the impedances at the notch frequencies of adjustable bandstop filters 2046 and 2048 to an output impedance approximating an opencircuit to avoid loading the output at these notch frequencies.

Lastly, tunable band stop filters 2054 and 2056 of the receive path aretuned to provide a notch at the transmit frequency of the first transmitpath and a notch at the transmit frequency of the second transmit path.Adjustable phase shifter/matching network 2058 is configured totransform the impedances at the notch frequencies of adjustable bandstop filters 2054 and 2056 to an output impedance approximating an opencircuit to avoid loading the output at these notch frequencies. In someembodiments, adjustable phase shifters matching networks 2040, 2050 and2058 each include two adjustable phase shifter/matching networks coupledin series that are each configured to provide an impedancetransformation for a single notch frequency.

By having band stop filters in each signal path that notch out sensitivefrequencies of the remaining signal paths, interference between signalpaths is attenuated and/or significantly reduced. It should beunderstood that the embodiments of FIG. 20A and FIG. 20B can be extendedto any number of transmit and/or receive signal paths.

Embodiments of the present invention can also be extended to timedivision duplex (TDD) systems as is illustrated in 21A and 21B. Invarious embodiments, band stop filters are used to notch out criticalfrequencies used by other transceiver systems that share a same antennaor have antennas that are in close proximity to the embodiment TDDsystem. As shown in FIG. 21A, RF system 2100 includes an RF transceiver2102 having a transmit output TX and a receive input RF. A transmit pathincludes a power amplifier 2104 having an input coupled to the transmitoutput TX of RF transceiver 2102, tunable band stop filter 2106 coupledto the output of power amplifier 2104 and a transmission switch 2108. Areceive signal path includes an LNA 2112 having an input coupled totransmission switch 2108 and an output coupled to the receive input ofRF transceiver 2102. An optional tunable filter 2114 may be coupledbetween transmission switch 2108 and LNA 2112.

During operation, transmission switch 2108 selectively couples one ofthe receive path or transmit path to antenna 2110 depending on whetherRF transceiver 2102 is transmitting or receiving. For example, when RFtransceiver 2102 is transmitting, the transmit path is coupled toantenna 2110 via transmission switch 2108. On the other hand, when RFtransceiver 2102 is receiving, the receive path is coupled to antenna2110 via transmission switch 2108. An optional second transceiver system2118 is also coupled to antenna 2110 and may be transmitting and/orreceiving at the same time as RF transceiver 2102. In some embodiments,optional second transceiver system 2118 may include more than onetransceiver and/or more than one receive and/or transmit paths. Theinterface 2116 between optional second transceiver system 2118 and therest of the system is depicted as a cloud, and may be implemented as adirect connection to transmission switch 2108 and antenna 2110 or may beimplemented using other coupling circuits known in the art.

In various embodiments, tunable band stop filter 2106 coupled to theoutput of power amplifier in the transmission path may be tuned to acritical frequency of RF system 2100. For example, in a dualconnectivity system, the frequency of tunable band stop filter 2106 maybe tuned to a receive frequency of the optional second transceiversystem 02118. In an uplink carrier aggregation (ULCA) system, thefrequency of tunable band stop filter 2106 may be tuned to a transmitfrequency of optional second transceiver system 2118. In addition,optional receive filter 2114 may also be tuned and/or configured toreject frequencies generated by optional second transceiver system. Infurther embodiments, tunable band stop filter 2106 may be implementedusing a plurality of tunable band stop filters that are each tuned to adifferent critical frequency of RF system 2100. Similarly, optionalreceive filter 2114 may be configured to reject different criticalfrequencies of RF system 2100. While FIG. 20A illustrates a systemhaving two transmit signal paths, it should be understood that in someembodiments, RF system 2000 may include more than two transmit signalspaths.

FIG. 21B illustrates RF system 2130 that is similar to RF system 2100 ofFIG. 21A, but further includes an additional band stop filter 2132coupled between the transmit output TX of RF transceiver 2102 and theinput of power amplifier 2104. In various embodiments, band stop filter2132 may be configured to reject one or more critical frequenciesreceived and/or generated within RF system 2130, including frequenciesreceived and generated by optional second transceiver system 2118. Insome embodiments, tunable band stop filter 2106 coupled to the output ofpower amplifier 2104 may be omitted to save power. While FIG. 20Billustrates a system having two transmit signal paths and one receivesignal path, it should be understood that in some embodiments, RF system2130 may include more than two transmit signals paths and/or may includemore than one receive signal paths.

In various embodiments, the tunable filters, including tunable band stopfilter, tunable bandpass filters and tunable phase shifter/matchingnetworks depicted in FIGS. 17A-C, 18A-D, 19A-C, 20A-B and 21A-B can beimplemented using tunable filter structures known in the art.

In various embodiments, the tunable band stop filters depicted in FIGS.17A-C, 18A-D, 19A-C, 20A-B and 21A-B may be implemented using tunableband stop filter structures known in the art or may be implemented usingtunable acoustic filter structures disclosed herein. For example, theacoustic filter structures shown in FIGS. 7A-7N, 8A-8E, 9A-9C, and11A-11C may be used. In some embodiments, these band stop filters may beimplemented using continuously tunable circuits. Tunable bandpass filtermay be implemented using circuits known in the art, continuously tunablecircuits, and/or tunable acoustic filter based bandpass structuresillustrated in FIGS. 9A-9C, 10A-10G, 11A-11C, 12A-12E and 13A-13E.

The various adjustable phase shifter/matching network circuits depictedin FIGS. 18A-D and 20A-B may be implemented using adjustable phaseshifter/matching network circuits known in the art such phase shiftingcircuit based on varactors and switched capacitors. The depictedadjustable phase shifter/matching network circuits may also beimplemented using tunable bridged T all-pass circuits depicted in FIGS.7A-7N above.

Advantages of the embodiments depicted in FIGS. 17A-C, 18A-D, 19A-C,20A-B and 21A-B include reduced transmit path power consumption,increased transmit spectral emission purity, improved receive path noisefigure to increase reference sensitivity, reduction in complexity due toa reduced number of filter components (and reduced filter order),reduction in system and PCB size due to the reduced number ofcomponents, and reduced system cost due to reduced component count,reduced system complexity and reduced PCB size.

As shown, for example, in FIGS. 18A-D, 20A-B and 21A-B above, tunablefilters at the output of a transmit path and/or at the input of areceive path of an RF system can be used to replace multiple transmit,receive and/or duplex filters. This concept can be generalized andextended to multiple systems, implementations and architectures as shownin FIG. 22, which illustrates a table depicting transmit/receivepath/combining structures A, B, C, D and E on the left hand side of thetable, and transmit/receive path filter configurations I, II, III, IV,V, VI, VII, VIII, IX, X, XI and XII on the top side of the table. Inembodiments of the present invention, the transmit/receivepath/combining structures A, B, C, D and E can be combined withtransmit/receive path filter configurations I, II, III, IV, V, VI, VII,VIII, IX, X, XI and XII to form various embodiment transmit/receive pathimplementations. Each of transmit/receive path/combining structures A,B, C, D and E includes a receive path and a transmit path that can becoupled to the respective transmit path output and receive path inputterminals of an RF transceiver such as those depicted in the Figuresabove.

Transmit/receive path/combining structure A depicts a structure thatincludes a transmit path (TX path), and receive path (RX path), atransmit/receive switch, an RF filter H(f) and an antenna.Transmit/receive path/combining structure A may be used, for example, ina TDD system in which the transmit/receive switch couples the transmitpath (TX path) to the antenna during signal transmission, and coupledthe receive path (RF path) to the antenna during signal reception. Thefilter structures depicted under the headings A_(I), A_(II), A_(V),A_(VI), A_(VII), A_(IX), A_(X), and A_(X) represent differentconfigurations that may be used to implement RF filter H(f). Forexample, filter H(f) may be implemented using filter configuration A_(I)representing a tunable band stop filter, which may be used to suppresssignals at a transmit frequency of another transmitter in the system toprovide lower noise in the receive path. Filter configuration A_(II)represents a tunable bandpass filter that may be tuned to the transmitfrequency and/or the receive frequency in order to suppress out of bandinterferers. Filter configuration A_(V) represents a fixed band-rejectfilter that may be used to reject fixed frequency interferers, andfilter configuration A_(VI) represents a fixed band pass filter that maybe set to a have a center frequency that includes both the transmit andreceive frequencies.

Filter configuration A_(VIII) represents a tunable band stop filterfollowed by a fixed bandpass filter and filter configuration A_(X)represents a fixed bandpass filter followed by a tunable band stopfilter. In these configurations, the tunable band stop filter may beused to highly suppress other transmit frequencies generated by othertransmitters in the RF system, and the fixed bandpass filter may be usedto attenuate noise and interference outside of the passband of the bandpass filter in cases where the frequencies of the transmit and receivepath are close to each other or are identical.

Filter configuration A_(IX) representing a tunable band stop filterfollowed a tunable bandpass filter and filter configuration A_(XI)representing a tunable bandpass filter followed by a tunable band stopfilter may be used to highly suppress other transmit frequenciesgenerated by other transmitters in the RF system, as well as toattenuate noise and interference outside of the passband of the bandpass filter.

In various embodiments, transmit/receive path/combining structures B, C,D and E along with filter configurations I, II, III, IV, V, VI, VII,VIII, IX, X, XI and XII may be used to implement the receive andtransmit path in an FDD system that does not require the use of aduplexer or fixed RF filter banks. These combinations are designated inthe table by letter (A, B, C, D and E) and Roman numeral (I, II, III,IV, V, VI, VII, VIII, IX, X, XI and XII). For example, theimplementation of transmit/receive path/combining structures C withfilter configuration VII is designated as C_(VII), and theimplementation of transmit/receive path/combining structures E withfilter configuration I is designated as E_(I), etc.

Transmit/receive path/combining structure B includes a transmit pathhaving a filter H_(TX)(f) followed by an adjustable phaseshifter/matching network coupled to a shared antenna, and a receive pathhaving a filter H_(RX)(f) followed by another adjustable phaseshifter/matching network coupled to a shared antenna. Each adjustablephase shifter/matching network may be used to transform the stop bandimpedance of a filter in one signal path to an impedance approximatingan open circuit to the other signal path. For example, the adjustablephase shifter/matching network of the transmit path can transform thestop band impedance of filter H_(TX)(f) to an impedance approximating anopen circuit at the receive frequency of the receive path to avoidloading the input of the receive path and attenuating the receivesignal. Similarly, the adjustable phase shifter/matching network of thereceive path can transform the stop band impedance of filter H_(RX)(f)to an impedance approximating an open circuit at the transmit frequencyof the transmit path to avoid loading the transmit path. Transmit filterH_(TX)(f) and receive filter H_(RX)(f) can be implemented using one offilter configurations I, II, III, IV, V, VI, VII, VIII, IX, X, XI andXII as will be explained below. The combination of transmit filterH_(TX)(f) and receive filter H_(RX)(f) transfer functions and adjustablephase shifter/matching networks provides isolation between the transmitpath and receive paths.

In some embodiments, the output of the adjustable phase shifter/matchingnetwork in the transmit signal path coupled to the antenna can beconsidered to be a transmit antenna port, and the input of theadjustable phase shifter/matching network in the receive signal pathcoupled to the antenna can be considered to be a receive antenna port.In this case, both the transmit antenna port and the receive antennaport is coupled together.

Transmit/receive path/combining structure C includes a transmit pathhaving a filter H_(TX)(f) and a receive path having a filter H_(RX)(f).The transmit path and receive path are coupled to an antenna using an RFcirculator. In an embodiment, the circulator provides low insertion lossfor signals propagating from the transmit path to the antenna, and fromthe antenna to the receive path, but isolates signals propagating fromthe transmit path to the receive path, and from the receive path to theantenna. The circulator may be implemented, for example, usingcirculator structures known in the art, for example, structuresdisclosed in the following references H. Obiya, T. Wada, H. Hayafuji, T.Ogami, M. Tani, M. Koshino, M. Kawashima and N. Nakajima, “A New TunableRF Front-End Circuit for Advanced 4G Handsets”, 2014 IEEE MT-S Int.Microwave Symp. Digest, session WEP-54, June 2014; T. Ogami, M. Tani, K.Ikada, H. Kando, T. Wada, H. Obiya, M. Koshino, M. Kawashima and N.Nakajima, “A New Tunable Filter Using Love Wave Resonators forReconfigurable RF”, 2014 IEEE MIT-S Int. Microwave Symp. Digest, sessionTU3A-2, June 2014; and T. Wada, R. Nakajima, H. Obiya, T. Ogami, M.Koshino, M. Kawashima and N. Nakajima, “A Miniaturized Broadband LumpedElement Circulator for Reconfigurable Front-end System”, 2014 IEEE MIT-SInt. Microwave Symp. Digest, session WEP-28, June 2014, which referencesare incorporated by reference. In some embodiments, the circulator maybe turned and/or matched to the receive and transmit frequencies used bythe respective receive and transmit paths. Transmit filter H_(TX)(f) andreceive filter H_(RX)(f) can be implemented using one of filterconfigurations I, II, III, IV, V, VI, VII, VIII, IX, X, XI and XII aswill be explained below.

Transmit/receive path/combining structure D includes a transmit pathhaving an in-phase transmit path (TX path (I)) including in-phasetransmit filter H_(TXi)(f), a quadrature transmit path (TX path (Q))including quadrature transmit filter H_(TXi)(f), and a receive path (RXpath) including receive filter H_(RX)(f). Coupling between the antennaand the receive path, input-phase transmit path and quadrature transmitpath is achieved via four port quadrature combiner, which can beimplemented, for example using quadrature combiner structures and/orquadrature hybrid structures known in the art, such as a Fisher hybrid.As shown, the in-phase transmit path is coupled to the input port of thequadrature combiner, the quadrature transmit path is coupled to theisolated port of the quadrature combiner, the receive path is coupled tothe −45° port of the quadrature combiner and the antenna is coupled tothe +45° port of the quadrature combiner. During operation, the RFtransceiver (not shown) generates an in-phase transmit signal and aquadrature transmit signal at are 90° out of phase with each other. Insome embodiments, the in-phase transmit signal and a quadrature transmitsignal may be generated using a quadrature splitter/combiner circuit,such as a polyphase filter. Accordingly, combining structure D forms afour port hybrid filter arrangement that allows for good coupling fromthe transmit paths to the antenna and from the antenna to the receivepath, but provides isolation from the transmit paths to the receivepaths. Transmit filters H_(TXi)(f) and H_(TXq)(f) and receive filterH_(RX)(f) can be implemented using one of filter configurations I, II,III, IV, V, VI, VII, VIII, IX, X, XI and XII as will be explained below.

Transmit/receive path/combining structure E includes a transmit pathhaving a filter H_(TX)(f) followed by an adjustable phaseshifter/matching network coupled to a transmit antenna, and a receivepath having a filter H_(RX)(f) followed by another adjustable phaseshifter/matching network coupled to a separate receive antenna. Eachadjustable phase shifter/matching network may be used to transform thestop band impedance of a filter in one signal path to an impedanceapproximating an open circuit (or other impedance) to its respectiveantenna. Providing this higher impedance to potentially interferingsignals further reduces the amount of interfering signal energy coupledto the respective signal path. Both adjustable phase shifters are tunedto provide maximum isolation between both antennas in some embodiments.Transmit filter H_(TX)(f) and receive filter H_(RX)(f) can beimplemented using one of filter configurations I, II, III, IV, V, VI,VII, VIII, IX, X, XI and XII as will be explained below.

In some embodiments, the output of the adjustable phase shifter/matchingnetwork in the transmit signal path coupled to the antenna can beconsidered to be a transmit antenna port, and the input of theadjustable phase shifter/matching network in the receive signal pathcoupled to the antenna can be considered to be a receive antenna port.In this case, unlike transmit/receive path/combining structure Ediscussed above, the respective receive and transmit antenna ports arecoupled to separate antennas.

As mentioned above each filter H_(TX)(f) and receive filter H_(RX)(f)can be implemented using one of filter configurations I, II, III, IV, V,VI, VII, VIII, IX, X, XI and XII depicted in FIG. 22. Filterconfiguration I implements transmit filter H_(TX)(f) in the transmitpath as a tunable band stop filter that is tuned to the receivefrequency in order to suppress transmit noise at the receive frequency,and implements receive filter H_(RX)(f) in the receive path as a tunableband stop filter that is tuned to the transmit frequency in order tosuppress the transmit signal during FDD operation.

Filter configuration II implements transmit filter H_(TX)(f) in thetransmit path as a tunable bandpass filter that is tuned to the transmitfrequency in order to suppress transmit noise and transmit spuriousemissions, and implements receive filter H_(RX)(f) in the receive pathas a tunable bandpass filter that is tuned to the receive frequency inorder to suppress one or more transmit signals and/or out-of-of bandinterferers.

Filter configuration III implements transmit filter H_(TX)(f) in thetransmit path as a tunable bandpass filter that is tuned to the transmitfrequency in order to suppress transmit noise and transmit spuriousemissions, and implements receive filter H_(RX)(f) in the receive pathas a tunable band stop filter that is tuned to the transmit frequency inorder to suppress the transmit signal during FDD operation.

Filter configuration IV implements transmit filter H_(TX)(f) in thetransmit path as a tunable band stop filter that is tuned to the receivefrequency in order to suppress transmit noise at the receive frequency,and implements receive filter H_(RX)(f) in the receive path as a tunablebandpass filter that is tuned to the receive frequency in order tosuppress one or more transmit signals and/or out-of-of band interferers.

Filter configuration V implements transmit filter H_(TX)(f) in thetransmit path as a fixed band stop filter that is tuned to the receivefrequency in order to suppress transmit noise at the receive frequency,and implements receive filter H_(RX)(f) in the receive path as a fixedband stop filter that is tuned to the transmit frequency in order tosuppress the transmit signal during FDD operation.

Filter configuration VI implements transmit filter H_(TX)(f) in thetransmit path as a fixed bandpass filter that is tuned to the transmitfrequency in order to suppress transmit noise and transmit spuriousemissions, and implements receive filter H_(RX)(f) in the receive pathas a tunable band stop filter that is tuned to the transmit frequency inorder to suppress the transmit signal during FDD operation.

Filter configuration VII implements transmit filter H_(TX)(f) in thetransmit path as a tunable band stop filter that is tuned to the receivefrequency in order to suppress transmit noise at the receive frequency,and implements receive filter H_(RX)(f) in the receive path as a fixedbandpass filter that is tuned to the receive frequency in order tosuppress one or more transmit signals and/or out-of-of band interferers.

Filter configuration VIII implements transmit filter H_(TX)(f) in thetransmit path as a tunable band stop filter followed by a fixedfrequency bandpass filter. The tunable band stop filter is tuned to thereceive frequency in order to suppress transmit noise at the receivefrequency, and the fixed frequency bandpass filter to provide widebandattenuation. Receive filter H_(RX)(f) in the receive path is implementedas a tunable band stop filter followed by a fixed bandpass filter. Thetunable band stop filter is tuned to the transmit frequency in order tosuppress the transmit signal during FDD operation, and the fixedbandpass filter provides wideband attenuation.

Filter configuration IX implements transmit filter H_(TX)(f) in thetransmit path as a tunable band stop filter followed by a tunablebandpass filter. The tunable band stop filter is tuned to the receivefrequency in order to suppress transmit noise at the receive frequency,and the tunable bandpass filter is tuned to the transmit frequency.Receive filter H_(RX)(f) in the receive path is implemented as a tunableband stop filter followed by a tunable bandpass filter. The tunable bandstop filter is tuned to the transmit frequency in order to suppress thetransmit signal during FDD operation, and the tunable bandpass filter istuned to the receive frequency.

Filter configuration X has the same elements as filter configurationVIII, with the exception that the position of the bandpass and band stopfilters are swapped, thereby placing the tunable band stop filterscloser to the antenna port. Having the tunable band stop filters closerto the antenna port may be useful, for example, when used to implementthe filters of transmit/receive path/combining structure B in someembodiments because it allows for a short signal path between the bandstop filter and the adjustable phase shifter/matching network that isused to modify the impedance of the tunable band stop filter. The choiceof which filter to place closest to the antenna, however, depends of theimpedance of the particular filter being implemented as well as thedetails and specification of the system being implemented. Thus, in somecases, it may be advantageous to implement transmit/receivepath/combining structure B using filter configuration VIII instead.

Filter configuration XI has the same elements as filter configurationIX, with the exception that the position of the bandpass and band stopfilters are swapped, thereby placing the tunable band stop filterscloser to the antenna port. As mentioned above with respect to filterconfiguration X, having the tunable band stop filters closer to theantenna port may be useful, for example, when used to implement thefilters of transmit/receive path/combining structure B in some cases,because the different the impedance might allow a better implementationof the phase shifters (e.g., smaller size, better insertion loss, andhigher bandwidth). The choice of which filter to place closest to theantenna, however, depends of the impedance of the particular filterbeing implemented as well as the details and specification of the systembeing implemented. Thus, in some cases, it may be advantageous toimplement transmit/receive path/combining structure B using filterconfiguration IX instead.

Filter configuration XII has a similar structure as filter configurationVIII, with the exception that fixed bandpass filter in the transmit pathis removed. This configuration may be suitable for systems in which thefilter requirements in the transmit path are less stringent that thefilter requirements in the receive path. This particular filterconfiguration is advantageous in the sense that it provides filtering ata low cost with low insertion loss in the transmit path.

It should be understood that the RF systems represented by the variousembodiments depicted in FIG. 22 are just a subset of many possibleembodiment RF system topologies. In alternative embodiments of thepresent invention, additional filters may be added to the transmit andreceive paths and/or additional filter combinations and permutations maypossible.

FIGS. 23, 24A-D, 25, 26A-B and 27 illustrates specific examples ofembodiment systems summarized in the table of FIG. 22. For example, FIG.23 illustrates a TDD system 2300 that corresponds to transmit/receivepath/combining structure A using filter configuration II (A_(II)). Asshown, the TDD system 2300 depicted in FIG. 23 includes a transmit path(TX path), a receive path (RX path), a transmit/receive switch 2302, atunable bandpass filter 2304 and an antenna 2306. In such anembodiments, the tunable bandpass filter 2304 is shared between thetransmit path and the receive path. During operation, thetransmit/receive switch 2302 selectively couples the active channel tothe antenna. For example, when the transmit signal path is active, thetransmit/receive switch 2302 couples the transmit signal path to theantenna 2306, and when the receive path is active, the transmit/receiveswitch 2302 couples the receive signal path to the antenna 2306.

FIG. 24A illustrates an RF system 2400 that corresponds totransmit/receive path/combining structure B using filter configurationII (B_(II)). As shown, the transmit path (TX path) includes a tunablebandpass filter 2404 followed by adjustable phase shifter/matchingnetwork 2406 and antenna 2412. Similarly, the receive path (RX path)includes a tunable bandpass filter 2408 followed by adjustable phaseshifter/matching network 2410 and antenna 2412. In various embodiments,adjustable phase shifter/matching networks 2406 and 2410 transform thestop band impedance of its respective tunable bandpass filter to animpedance that approximates an open circuit at an active frequency ofthe adjacent RF signal path as is described below.

FIG. 24B illustrates a graph of the insertion loss of tunable bandpassfilter 2404 in the receive path of RF system 2400 depicted in FIG. 24A.As shown, tunable bandpass filter 2404 has a passband of between about1.80 GHz and about 1.94 GHz, which corresponds to a receive band.

In one embodiment, the transmit path is configured to provide atransmitted signal at about 2.10 GHz that corresponds to the stop bandof tunable bandpass filter 2404 in the receive path, tunable bandpassfilter 2404 in the transmit path is configured to have a centerfrequency of about 2.10 GHz, and adjustable phase shifter/matchingnetwork 2410 is configured to rotate the input impedance of tunabletransmit filter 2408 to an impedance approximating an open circuit atthe 2.10 GHz transmit frequency as is illustrated in the Smith chartsshown in FIGS. 24C and 24D.

The Smith chart of FIG. 24C illustrates the input impedance (s11) oftunable bandpass filter 2404 between 160 GHz at point 2422 and 2.10 GHzat point 2424. As shown, the input impedance of at 2.10 GHz at point2424 is capacitive. The Smith chart of FIG. 24D illustrates the inputimpedance (s11) of tunable bandpass filter 2404 coupled in series withadjustable phase shifter/matching network 2410 between 1.60 GHz at point2422 and 2.10 GHz at point 2424. As shown, adjustable phaseshifter/matching network 2410 transforms the input impedance of at 2.10GHz at point 2424 from being capacitive to approximating an opencircuit. Thus, tunable bandpass filter 2404 in the receive path can beconfigured not to load the transmit channel at the transmit frequency of2.10 GHz. It should be appreciated that the performance graphs shown inFIGS. 24B-D represent the performance of just one example embodiment,and different embodiments may perform differently.

FIG. 25 illustrates an RF system 2400 that corresponds totransmit/receive path/combining structure C using filter configurationII (C_(II)). As shown, the transmit path (TX path) includes a tunablebandpass filter 2502 followed by three-port circulator 2506 and antenna2508. Similarly, the receive path (RX path) includes a tunable bandpassfilter 2504 followed by three-port circulator 2506 and antenna 2508.

FIG. 26A illustrates an RF system 2500 that corresponds totransmit/receive path/combining structure D using filter configurationII (D_(II)). As shown, RF system 2500 includes an in-phase transmit path(I-Phase TX path) including tunable bandpass filter 2602, a quadraturetransmit path (Q-Phase TX path) including tunable bandpass filter 2604,and a receive path (RX path) including tunable bandpass filter 2606.Coupling between the antenna 2610 and the receive path, input-phasetransmit path and quadrature transmit path is achieved via quadraturecombiner 2608. As shown, the in-phase transmit path is coupled to theinput port of the quadrature combiner, the quadrature transmit path iscoupled to the isolated port of the quadrature combiner, the receivepath is coupled to the −45° port of the quadrature combiner and theantenna is coupled to the +45° port of the quadrature combiner. Duringoperation the RF transceiver (not shown) generates an in-phase transmitsignal and a quadrature transmit signal at are 90° out of phase witheach other, or an in-phase and quadrature signal are generated using aquadrature splitter/combiner circuit.

FIG. 26B illustrates a plot of selectivity and insertion loss of betweenports 2 and 1, and between ports 3 and 1 of RF system 2600 shown in FIG.26A. Trace 2620 represents the selectivity between ports 2 and 1(transmit path); trace 2622 represents the insertion loss between ports2 and 1 (transmit path); trace 2624 represents the selectivity betweenports 3 and 1 (receive path); and trace 2626 represents the insertionloss between ports 3 and 1 (receive path).

As can be seen in FIG. 26B, the receive path whose selectivity isrepresented by trace 2620 has a passband between about 1.8 GHz and about1.92 GHz, while the transmit path whose selectivity is represented bytrace 2624 has a passband between about 2.2 GHz and about 2.18 GHz. Itshould be appreciated that the performance graphs shown in FIG. 26Drepresents the performance of just one example embodiment and differentembodiments may perform differently.

FIG. 27 illustrates an RF system 2700 that corresponds totransmit/receive path/combining structure E using filter configurationII (E_(II)). As shown, the transmit path (TX path) includes a tunablebandpass filter 2702 followed by adjustable phase shifter/matchingnetwork 2704 and transmit antenna 2706. Similarly, the receive path (RXpath) includes a tunable bandpass filter 2708 followed by adjustablephase shifter/matching network 2710 and separate receive antenna 2712.In various embodiments, adjustable phase shifter/matching networks 2704and 2710 transform the stop band impedance of its respective tunablebandpass filter to an impedance that approximates an open circuit at anactive frequency of the adjacent RF signal path.

In various embodiments, the tunable band stop filters depicted in FIGS.17A-C, 18A-D, 19A-C, 20A-B and 21A-B may be implemented using tunableband stop filter structures known in the art or may be implemented usingtunable acoustic filter structures disclosed herein. For example, theacoustic filter structures shown in FIGS. 7A-7F, 8A-8E, 9A-9C, and11A-11C may be used. In some embodiments, these band stop filters may beimplemented using continuously tunable circuits. Tunable bandpass filtermay be implemented using circuits known in the art, continuously tunablecircuits, and/or tunable acoustic filter based bandpass structuresillustrated in FIGS. 9A-9C, 10A-10G, 11A-11C, 12A-12E and 13A-13E.

The various adjustable phase shifter/matching network circuits depictedin FIGS. 18A-D and 20A-B may be implemented using adjustable phaseshifter/matching network circuits known in the art such phase shiftingcircuit based on varactors and switched capacitors. The depictedadjustable phase shifter/matching network circuits may also beimplemented using tunable bridged T all-pass circuits depicted in FIGS.7A-7F above.

Example embodiments of the present invention are summarized here. Otherembodiments can also be understood from the entirety of thespecification and the claims filed herein.

Example 1

A method of operating an RF system, the method including: filtering afirst wideband RF signal using a wideband filter bank, filtering thefirst RF signal including separating the first wideband RF signal intofrequency cluster signals, where each frequency cluster signal of thefrequency cluster signals includes different frequency ranges, the firstwideband RF signal includes multiple RF bands, and each of the differentfrequency ranges includes a plurality of RF bands of the multiple RFbands; band stop filtering at least one of the frequency cluster signalsto produce a band stopped frequency cluster signal.

Example 2

The method of example 1, further including amplifying the band stoppedfrequency cluster signal.

Example 3

The method of example 2, where amplifying the band stopped frequencycluster signal includes using a low noise amplifier (LNA).

Example 4

The method of one of examples 1 to 3, where band stop filtering includesusing a tunable band stop filter, and the method includes adjusting acenter frequency of the tunable band stop filter.

Example 5

The method of one example 4, where the first wideband RF signal includesan interfering signal at a first interfering frequency, and adjustingthe center frequency of the tunable band stop filter includes adjustingthe center frequency to first interfering frequency.

Example 6

The method of one of examples 1 to 5, where the wideband filter bankincludes a plurality of cascaded diplexers.

Example 7

The method of one of examples 1 to 6, where the wideband filter bankincludes an n-plexer.

Example 8

The method of one of examples 1 to 7, further including receiving thefirst wideband RF signal from an antenna.

Example 9

An RF system including: a wideband filter bank including an input and aplurality of outputs, the wideband filter bank configured to separate awideband RF signal at an input of the wideband filter bank into aplurality of frequency clusters at the plurality of outputs of thewideband filter bank, where each frequency cluster of the plurality offrequency clusters includes a different frequency range, and eachfrequency range covers a plurality of RF bands of the wideband RFsignal; and at least one band stop filter having an input coupled to oneof the plurality of outputs of the wideband filter bank.

Example 10

The RF system of example 9, further including an amplifier having aninput coupled to an output of the band stop filter.

Example 11

The RF system of example to, where the amplifier includes a low noiseamplifier (LNA).

Example 12

The RF system of one of examples 9 to 11, where the band stop filterincludes a tunable band stop filter.

Example 13

The RF system of one of examples 9 to 12, where a center frequency ofthe band stop filter corresponds to a frequency at which the wideband RFsignal includes an interfering signal.

Example 14

The RF system of one of examples 9 to 13, where the wideband filter bankincludes a plurality of cascaded diplexers.

Example 15

The RF system of example 14, where the wideband filter bank furtherincludes an n-plexer having an input coupled to an output of theplurality of cascaded diplexers.

Example 16

The RF system of one of examples 9 to 15, where the wideband filter bankincludes an n-plexer.

Example 17

An RF system including: a first RF filter having a first inputconfigured to be coupled to an antenna, the first RF filter configuredto provide a first bandpass response passing a first frequency band fromthe first input to a first bandpass output, and a first band stopresponse rejecting the first frequency band from the first input to afirst band stop output; an n-plexer having an input coupled to the firstband stop output of the first RF filter; and a first tunable band stopfilter coupled to an output of the n-plexer.

Example 18

The RF system of example 17, further including: a second RF filterhaving a second input coupled to the first band stop output of the firstRF filter, and a second band stop output coupled to the input of then-plexer, the second RF filter configured to provide a second bandpassresponse passing a second frequency band from the first input to asecond bandpass output, and a second band stop response rejecting thesecond frequency band from the second input to a second band stopoutput.

Example 19

The RF system of example 18, further including: a first low noiseamplifier (LNA) having an input coupled to the first bandpass output ofthe first RF filter; a second LNA having an input coupled to the secondbandpass output of the second RF filter; and a third LNA having an inputcoupled to an output of the first tunable band stop filter.

Example 20

The RF system of one of examples 17 or 18, where the first bandpassoutput of the first RF filter is configured to pass WiFi frequencies,and the second band stop output of the second RF filter and the outputof the n-plexer is configured to pass cellular communicationfrequencies.

Example 21

The RF system of example 20, further including a bypass switch coupledbetween the input of the first RF filter and the first band stop outputof the first RF filter, the bypass switch configured to be closed when aWiFi receiver of the RF system is deactivated.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method of operating an RF system, the methodcomprising: filtering a first wideband RF signal using a wideband filterbank, filtering the first RF signal comprising separating the firstwideband RF signal into frequency cluster signals, wherein eachfrequency cluster signal of the frequency cluster signals comprisesdifferent frequency ranges, the first wideband RF signal comprisesmultiple RF bands, and each of the different frequency ranges comprisesa plurality of RF bands of the multiple RF bands; band stop filtering atleast one of the frequency cluster signals to produce a band stoppedfrequency cluster signal.
 2. The method of claim 1, further comprisingamplifying the band stopped frequency cluster signal.
 3. The method ofclaim 2, wherein amplifying the band stopped frequency cluster signalcomprises using a low noise amplifier (LNA).
 4. The method of claim 1,wherein band stop filtering comprises using a tunable band stop filter,and the method further comprises adjusting a center frequency of thetunable band stop filter.
 5. The method of claim 4, wherein the firstwideband RF signal comprises an interfering signal at a firstinterfering frequency, and adjusting the center frequency of the tunableband stop filter comprises adjusting the center frequency to firstinterfering frequency.
 6. The method of claim 1, wherein the widebandfilter bank comprises a plurality of cascaded diplexers.
 7. The methodof claim 1, wherein the wideband filter bank comprises an n-plexer. 8.The method of claim 1, further comprising receiving the first widebandRF signal from an antenna.
 9. An RF system comprising: a wideband filterbank comprising an input and a plurality of outputs, the wideband filterbank configured to separate a wideband RF signal at an input of thewideband filter bank into a plurality of frequency clusters at theplurality of outputs of the wideband filter bank, wherein each frequencycluster of the plurality of frequency clusters comprises a differentfrequency range, and each frequency range covers a plurality of RF bandsof the wideband RF signal; and at least one band stop filter having aninput coupled to one of the plurality of outputs of the wideband filterbank.
 10. The RF system of claim 9, further comprising an amplifierhaving an input coupled to an output of the band stop filter.
 11. The RFsystem of claim 10, wherein the amplifier comprises a low noiseamplifier (LNA).
 12. The RF system of claim 9, wherein the band stopfilter comprises a tunable band stop filter.
 13. The RF system of claim9, wherein a center frequency of the band stop filter corresponds to afrequency at which the wideband RF signal comprises an interferingsignal.
 14. The RF system of claim 9, wherein the wideband filter bankcomprises a plurality of cascaded diplexers.
 15. The RF system of claim14, wherein the wideband filter bank further comprises an n-plexerhaving an input coupled to an output of the plurality of cascadeddiplexers.
 16. The RF system of one claim 9, wherein the wideband filterbank comprises an n-plexer.
 17. An RF system comprising: a first RFfilter having a first input configured to be coupled to an antenna, thefirst RF filter configured to provide a first bandpass response passinga first frequency band from the first input to a first bandpass output,and a first band stop response rejecting the first frequency band fromthe first input to a first band stop output; an n-plexer having an inputcoupled to the first band stop output of the first RF filter; and afirst tunable band stop filter coupled to an output of the n-plexer. 18.The RF system of claim 17, further comprising: a second RF filter havinga second input coupled to the first band stop output of the first RFfilter, and a second band stop output coupled to the input of then-plexer, the second RF filter configured to provide a second bandpassresponse passing a second frequency band from the first input to asecond bandpass output, and a second band stop response rejecting thesecond frequency band from the second input to the second band stopoutput.
 19. The RF system of claim 18, further comprising: a first lownoise amplifier (LNA) having an input coupled to the first bandpassoutput of the first RF filter; a second LNA having an input coupled tothe second bandpass output of the second RF filter; and a third LNAhaving an input coupled to an output of the first tunable band stopfilter.
 20. The RF system of claim 18, wherein the first bandpass outputof the first RF filter is configured to pass WiFi frequencies, and thesecond band stop output of the second RF filter and the output of then-plexer is configured to pass cellular communication frequencies. 21.The RF system of claim 20, further comprising a bypass switch coupledbetween the input of the first RF filter and the first band stop outputof the first RF filter, the bypass switch configured to be closed when aWiFi receiver of the RF system is deactivated.